Colonel Herman L. Gilster

UNITED STATES Air Force doctrine defines three basic combat missions for tactical air power: counter air, close air support, and air interdiction.¹ Counter air operations are conducted to gain and maintain air supremacy by attacking the enemy's combat aircraft, air bases, antiaircraft artillery (AAA), and surface-to-air missile (SAM) sites. Essentially, these attacks are designed to provide all friendly aircraft the capability to operate freely in the airspace above both friendly and enemy territory. The second mission, close air support, encompasses the use of air power in direct support of friendly land forces. Close air support attacks are made against targets of urgent concern in the immediate battle area and require direct and effective integration between the friendly ground and air forces. Finally, air interdiction, the subject of this article, is defined as the systematic attack of enemy's logistics network for the purpose destroying, neutralizing, or delaying his military potential (manpower and materiel) before it can be brought to bear effectively against friendly ground forces. The range of interdiction strikes may span a distance from the immediate battlefield up to, and sometimes including, the enemy's heartland. Normally, these attacks are made at such a distance to the enemy's rear that detailed coordination with friendly ground forces is unnecessary.

Categorization of the functions of tactical air power into the three missions cited already should not conceal the fact that these missions are in no sense mutually exclusive. Nevertheless, mission definition is useful, in that it provides a point of departure for any discussion of the impact and effectiveness of major air components. For instance, it is relatively simple to determine the success of the counter air function by noting the ease or difficulty with which friendly aircraft operate overhead. Likewise, the impact of the close air support function can be evaluated with respect to the success or failure of the ground force it supports. Fortunately, these "measures of merit" are tangible, highly visible, and immediately apparent. Consequently, such operations are recognized as viable, productive missions of air power. True, the military services may debate the question of who can most effectively perform these missions, but there is no question of their importance or whether they fit into the spectrum of vital military operations.

The same cannot be said for the third mission, air interdiction. This mission, along with its effectiveness and viability, has been the subject of some of the most intense debates within civilian and military circles in the Department of Defense during the last ten years. This is not surprising because interdiction by its very nature may not carry with it an immediate payoff. In addition, it has been difficult to show, historically, a consistent payoff for the supply denial objective in terms of its impact on the outcome of a campaign. What is observed is merely the ability of the enemy to fight at the current operating level, a level which he may or may not have selected as a result of the burden imposed on him by air interdiction. Without knowledge of the enemy's precise intentions, one finds it virtually impossible to determine whether the interdiction effort seriously limited his capability to operate at a preferred level of activity. Indeed, some insight into the impact of interdiction during World War II has been gained from German records and interviews, but, barring a similar exchange, we will probably never be able to assess with certainty its true impact during the Korean and Southeast Asian conflicts.

World War II to Southeast Asia

Historical reviews of our experience with air interdiction have concluded that the most dramatic successes were recorded when air interdiction missions were complemented by aggressive ground operations on the part of friendly forces. Operation Strangle, the first full-scale, consciously planned interdiction campaign of World War II, is a prime example. Conducted from March through May of 1944 in Italy, this campaign was initially assigned the optimistic objective of forcing the withdrawal of the German armies from central Italy by denying them essential supplies. This objective was, of course, unrealistic. Only after the Allied ground offensive was launched on 11 May 1944 did the tangible effect of air interdiction become evident. Within three weeks, the four-month stalemate on the ground had been broken, and the German army was in full retreat. The enemy withdrew some 200 miles, suffering an estimated 70,000 casualties, about one-third of his force in Italy.

In an evaluation of this campaign, F. M. Sallagar of the Rand Corporation concluded that success of the Allied forces cannot be attributed to the accomplishment of the supply-denial objective. ² The enemy transportation network had an estimated capacity of over 90,000 tons per day while enemy requirements totaled much less than 5000 tons per day. The stocks of some critical items such as fuel (gasoline and diesel) and ammunition remained fairly level or actually increased during the pure interdiction phase. They declined later when German army consumption rose steeply during the Allied ground offensive but never to the point of creating overall shortages at the front. This is evident in the figures of Table 1, extract from the quartermaster records of the German army for three key dates: (1) 15 March—the start of Operation Strangle, (2) 11 May—the start of the ground offensive: and (3) 30 May—the beginning of the precipitate German retreat. 

Sallagar attributes the failure of interdiction to achieve the supply-denial objective the following factors, most of which were inherent in the tactical situation confronting the Allies and therefore beyond their control.

During Strangle, the major factors were the redundant capacity of the enemy’s transport network, especially in the north where the interdiction belt had been placed; German ingenuity in effecting quick repairs, finding alternative routes, and improvising substitutes, the frugal living standards and stringent conservation measures imposed on German armies, coupled with their low consumption rates during the two months while there was no ground action on the front; the intermittent periods of bad weather when Allied air was grounded so that the Germans were able to make repairs and move up supplies; and the lack of an adequate night bomber capability, which made the nighttime relatively safe for repair work and the movement of supplies. ³

If the above rationale sounds familiar, one should not be surprised. With the possible exception of the last factor, the same list been fundamental to debates on the viability of interdiction during the Korean and Vietnam conflicts where, as Sallagar states, "we faced an enemy who was definitely not road bound, whose consumption needs were frugal beyond anything the Germans ever dreamed of, to whom the holding of territory meant little, and who could select the time and occasion when he was willing to fight." ⁴

Table I. German army supply status during Operation Struggle (metric tons).

Source: F.M. Sallager, Operation "Strangle" (Italy, Spring 1944): A Case Study of Tactical Air Interdiction, The Rand Corporation, R-851-PR, February 1972.

Despite the obvious failure of Operation Strangle to achieve supply denial, Sallagar concluded that the interdiction effort deserved a major share of the credit for the Allied victory. Although interdiction did not achieve its stated objective, it contributed immeasurably to the defeat of the German armies by denying them the tactical mobility at was so essential to them. By the enemy's own testimony, the reduction and occasional paralysis of his freedom of movement contributed more than any other single factor to his defeat. The disruption effected by Allied air attacks overwhelmed the enemy's distribution system, and although the aggregate supply base was sufficient for combat operations, it was impossible for the Germans to position men and materiel at the right place the right time.

This same pattern—aggressive ground action that forces the enemy to expend men and materiel in battle, overlaid by systematic interdictive air strikes which limit his capability to bring the required replacements into action—has resulted in some of interdiction's most acclaimed successes. The classic ample of such a large-scale joint-force operation occurred preparatory to and during Operation Overlord, the Allied invasion of Normandy in 1944. The devastating impact air strikes during that campaign was best described by Field Marshal Karl von Rundstedt, Commander of the German Western Front:

After the first few days, I had no hopes of defeating the invasion. The Allied Air Forces paralyzed all movement by day, and made it very difficult even at night. They had smashed the bridges over the Loire as well as over the Seine, shutting off the whole area. These factors greatly delayed the concentration of reserves there—they took three or four times longer to reach the front than we had reckoned. ⁵

Despite the theoretical availability of the most elaborate and interconnected road and railroad network in the world, the German army was unable to match the Allies' cross channel rate of build-up in the battle area. This failure was in large part the result of air interdiction strikes.

Similar successes were recorded during the first year of the Korean conflict, when the United Nation's ground forces were actively engaged with the enemy. Starting in July 1951, however, when armistice negotiations were initiated, a new chapter in the history of air power was opened. As a result of the politically imposed military stalemate that lasted until the cease fire in July 1953, military commanders were confined in the use of air assets to a new, unfamiliar environment of protracted war. During the ensuing two-year period, a series of special purpose interdiction campaigns was waged on the railroad and highway network to the enemy's rear. Although each of these efforts met with initial success, the general consensus was that these successes were of fleeting nature.⁶ The flexibility of the enemy's logistics system, the ability of the enemy to effect rapid repairs, and the extremely low supply requirements resulting from little or no ground action militated against any lasting success that might have been visualized. Hence, there is no tangible evidence that interdiction significantly impaired the enemy's capability during the two-year stalemate, and without access to his intentions or records, we cannot confirm with certainty the failure or qualified success of the interdiction effort in Korea.

The resulting frustrations, doubts, and differences of opinion over the viability of air interdiction were further exacerbated during the recent Southeast Asian conflict. Debate raged hot and heavy over the continued support of this expensive but questionable mission. This, of course, was no moot exercise since over one-half of all combat sorties flown during World War II, Korea, and Southeast Asia were allocated to interdiction operations.

Few experts question the viability of the "tactical" variety of air interdiction which can be closely related to battlefield success. Rather, it is the viability of the "long-term supply denial" version, which characterized U.S. air efforts during lengthy phases of the protracted conflict in Southeast Asia, that has been questioned. Although examples of the former are included, the main thrust of this article is directed toward the latter form of interdiction. In particular, it concentrates on an evaluation of the air interdiction campaigns waged for three and a half years in southern Laos. Not only did these campaigns receive the most extensive quantitative documentation of the war but they also provide the purest example of our experience with air interdiction in a protracted conflict.

Air Operations in Southern Laos

Although bombing operations had been initiated earlier, the first full-season interdiction campaign in Southeast Asia was conducted during the summer of 1966 in an interdiction belt across the lower panhandle of North Vietnam.⁷ In the summer of 1967, the weight of effort shifted north to the enemy’s heartland for the purpose of destroying North Vietnamese military and industrial facilities and paralyzing the railroads. The campaign against the heartland was continued until the 1 April 1968 bombing halt again restricted strike operations to the lower panhandle. Then, on 1 November 1968, President Johnson halted all bombing of North Vietnam.

As the result of a contingent agreement with North Vietnam that prohibited movement of men and materiel directly through the demilitarized zone between North and South Vietnam, attention immediately shifted to the Ho Chi Minh Trail in the panhandle of southern Laos, where the majority of enemy supplies moving from north to south now traversed. Thus began a series of dedicated, interdiction campaigns, code-named Commando Hunt, that continued until the North Vietnamese invasion of South Vietnam in the spring of 1972. Strikes against the trail had been conducted earlier, but these were generally considered secondary to attacks on primary targets in North Vietnam. The official beginning of the concerted interdiction effort in southern Laos was 1 November 1968.

The geographic and climatic features of southern Laos conditioned all aspects of campaign planning, operations, and results. Prominent among the geographic features is the Annam Mountain Range, which forms a natural boundary between Laos and North Vietnam. It is rugged and difficult to traverse, and vehicular entry to Laos is possible only at the major passes. The roads through the passes, however, are normally concealed in clouds, and beyond the passes the tropical forests of Laos provide an almost continuous roof of natural concealment, severely inhibiting both the detection and destruction of targets from the air.

A second critical feature is the climate that is dominated by two major seasonal phenomena—the southwest and northeast monsoons. The southwest monsoon normally predominates from June to October and the northeast from November to May. The climatological patterns for each of the seasons are best remembered with reference to the Annam Mountains. During the southwest monsoon, or wet season, a low-pressure area draws air off the Indian Ocean, bringing thunderstorms and rains to Laos. During the northeast monsoon, or dry season, a high-pressure area blows over the Gulf of Tonkin and South China Sea, bringing low overcast clouds, fog, and drizzle to North Vietnam and dry weather to Laos.

The shifting nature of the monsoons had an important bearing on the interdiction effort because the enemy geared his logistics flow to it. The northeast monsoons brought improved weather conditions over the roads and made them much more suitable for the movement of men and supplies. Consequently, the enemy concentrated his logistics efforts during these periods, and the interdiction campaigns were planned to respond accordingly.

These, then, are the characteristic features of the famed Ho Chi Minh Trail, which served as the primary artery for moving North Vietnamese supplies into South Vietnam. The trail's history as a line of communication (LOC) dated back to World War II, when Vietminh bands trekked the same jungle paths. This LOC was developed from existing footpaths into a highly organized infiltration route for men and supplies. The road network extended from Mu Gia Pass in the north, southward along the heavily forested western slopes of the Annam range, to a series of exit points stretching from just below the demilitarized zone between the two Vietnams, to the triborder region of Laos, Cambodia, and South Vietnam—some 500 kilometers to the south. (See Figure 1.) Although the road net was initially confined to the western slopes of the Annam range continued expansion of the system pushed additional miles of motorable routes further westward in Laos, providing the enemy an increasingly wide choice of routes along which he could channel supplies. By the summer of 1971, this labyrinth of routes and bypasses encompassed an estimated 3500 kilometers of motorable roads.

In spite of constant improvement, the roads were still primitive by Western standards, consisting primarily of 18-foot-wide tracks carved out of the jungles. Although both gravel and corduroy surfaces were used to strengthen some sections, the roads were chiefly dirt and nearly impassable during the wet season. The roads were originally built by manual labor, but as time passed on, the North Vietnamese made increased use of bulldozers, roadgraders, and other heavy equipment. The route network was operated, maintained, and defended by an estimated 40,000-50,000 personnel organize in geographic area units called Binh Trams. Each Binh Tram had the necessary transportation, engineer, and AAA battalions to e sure movement and security of materiel personnel in its sector.

The process by which supplies were moved southward was extremely complicated, requiring coordination between various transportation elements and numerous transfers of cargo in and out of vehicles and wayside storage areas. Almost all movement as conducted at night in a series of short shuttles, rather than by long-distance hauling. Drivers drove their trucks over the same routes night after night, becoming thoroughly familiar with their assigned segments. Periods of high moon illumination, which allowed travel without headlights, and low cloud cover were exploited to avoid detection from overhead aircraft. Truck movement began shortly after nightfall and normally trailed off about 3:00 a.m. to allow time for the unloading, dispersal, and concealment of supplies and vehicles before daylight. These tactics, developed in Korea and later refined in Laos, might be considered highly inefficient by Western standards, yet they were the most effective way of moving large quantities of supplies through a hostile environment.

Although the North Vietnamese later made limited use of waterways and pipelines, their road network and trucks remained throughout the war the heart of their logistics system. Intelligence estimates put the North Vietnamese truck inventory in Laos alone at 2500-3000 during the 1970 and 1971 dry seasons with from 500-1000 moving per night, each carrying about four tons of supplies. Replacement trucks were drawn from large inventories maintained within sanctuary of North Vietnam in the vicinity of Hanoi and Haiphong. During the height the interdiction campaigns, the trail logistics system was defended against U.S. aircraft with an estimated 600-700 antiaircraft guns.

On the U.S. side, a unique feature that distinguished the Commando Hunt campaigns from all previous interdiction campaigns was an electronic detection system that overlaid enemy logistics network with seismic and acoustic sensors. These sensors were air-delivered devices that detected enemy activity by noting acoustic or seismic disturbances within the range of the sensor. They were delivered by fighter aircraft in strings of six to eight beside known routes. Each sensor contained a self-destruct feature that was activated by a timer or an antitamper device. 

The sensor activations were received by orbiting aircraft and relayed to the Infiltration Surveillance Center, where they were, analyzed and translated into truck movements. These movements then became the basic index of enemy truck activity. This information was used on a real-time basis to position the interdiction force and on a longer-time basis to analyze trends, compute enemy input and throughput supply tonnages, and assist in the location of truck parks, storage areas, and new roads.

the Commando Hunt campaigns

The Commando Hunt interdiction campaigns carried numerical designators that changed with the semiannual monsoon shifts. Odd numbers designated the high-activity/dry season campaigns and even numbers the low-activity/wet season campaigns. Naturally, the dry season campaigns, conducted officially from November to May, received the most attention and study. Enemy logistics activity in southern Laos during the intervening wet seasons was so low that the corresponding military operations could hardly be classified as campaigns.

During the next dry season campaign, Commando Hunt III, the North Vietnamese logistics push during January and February reached new heights and was probably the most intense of the whole war. This effort, which netted a campaign throughput/input ratio of 1/3, may have been inspired by an anticipated loss of the alternate North Vietnamese LOC through Cambodia. Indeed, as the Commando Hunt III campaign was ending, the Cambodians did deny the North Vietnamese use of the port of Kompong Som, through which a large volume of material had been flowing. In addition, the Allied crossborder penetration into Cambodia during May and June further compounded the North Vietnamese difficulties: large quantities of food and ammunition that had been available to support forces in the southern regions of South Vietnam were lost. Subsequently, the North Vietnamese became actively engaged with Cambodian government in forces in operations that further increase their requirement for supplies from North Vietnam.

As a result, the Ho Chi Minh Trail assumed even greater significance as a LOC for enemy men and materiel. With the loss of Kompong Som and the supply line through Cambodia, the trail became not only the supply route for North Vietnamese and Vietcong forces in northern South Vietnam but also the main channel for resupply of enemy forces in southern South Vietnam and Cambodia. Although some leakages through other areas were possible, the Ho Chi Minh Trail remained the last major logistics avenue for the transport of supplies from north to south as the Commando Hunt V campaign approached.

Commando Hunt V was officially inaugurated on 10 October, three weeks early, to seize the initiative prior to the enemy's logistics push into Laos, which, according to intelligence estimates, was to begin on 14 October. The campaign was highlighted by a sustained, concentrated bombing effort in the entry passes to delay and impede traffic flow from October to January, followed by direct air support of the South Vietnamese ground incursion into Laos in February and March, all overlaid with an intensive truck-killing operation throughout southern Laos. More than 20,000 trucks, double the number of Commando Hunt III, were reported destroyed or damaged, and of the estimated 61,000 tons of supplies brought into Laos from North Vietnam, only 7000 tons reached Cambodia and South Vietnam--a throughput/input ratio of 1/9.¹⁰

The next dry season campaign, Commando Hunt VII, was inaugurated as usual during the month of November. U.S. forces averaged 182 fighter-attack, 13 gunship, and 21 B-52 sorties per day and reported destroying or damaging some 10,000 trucks through the end of March. The estimated throughput input ratio was running at a respectable 1/6 –5000 tons output for 31,000 tons input—when the enemy initiated a major invasion of South Vietnam over the Easter weekend at the end of March. Commando Hunt VII was immediately terminated, and the air resources that had been used on the trail were shifted to close air support and tactical interdiction roles within South Vietnam.

The Estimated Results

The 1972 enemy invasion of South Vietnam brought into question again the overall effectiveness of the interdiction effort in Southeast Asia and leads us back to the beginning. What was the impact of air interdiction on the Communist capability to fight in South Vietnam? Unfortunately, no firm quantitative conclusion on the viability of the interdiction campaigns can be advanced. Unlike World War II, there are no supply records or interviews with knowledgeable persons available for assessing true enemy desires and the impact of interdiction on the fulfillment of those desires.

One can only speculate with the use of estimates that may not be completely accurate. Supply tonnages, such as throughput and the enemy's minimum daily logistics requirements in South Vietnam, were routinely estimated, but intelligence analysts admit that these values could be off by a factor of two. Cumulating these values over several years adds even another dimension of uncertainty if reporting consistency has not been maintained from campaign to campaign. So although the values presented are best estimates, one should not attribute high accuracy to the absolute stock levels and requirements outlined in the following paragraphs.

Figure 2 gives a profile of estimated amounts of supplies that reached the borders of South Vietnam and Cambodia from the initiation of the Commando Hunt campaigns in November 1968 to the enemy invasion of South Vietnam in March 1972. The seasonal nature of the North Vietnamese logistics effort is readily apparent as is the major supply offensive during Commando Hunt III. It is interesting to note, however, that enemy combat activity in South Vietnam decreased continually throughout this period, including Commando Hunt III, until the major invasion in the spring of 1972. For example, enemy attacks by fire averaged 216 per month during Commando Hunt I, 138 during Commando Hunt III, and 88 during Commando Hunt V. Although some analysts have attempted to relate throughput tonnages with subsequent enemy activity in South Vietnam, there appears to be no correlation between the two. In fact, if one compares data from the Commando Hunt III and VII campaigns, a negative correlation would be implied, even though in the northern region of South Vietnam much of the invasion support flowed concurrently through the demilitarized zone and was not the result of a preinvasion effort along the Ho Chi Minh Trail.

Throughput to South Vietnam and Cambodia, of course, is only half the picture. To determine the enemy's supply status, we must also know something of his basic daily logistics requirements to survive and maintain current activity levels. The enemy’s minimum requirements were calculate monthly by intelligence analysts of the U.S. Military Assistance Command, Vietnam (MACV), and were predicated on estimated enemy strengths, consumption rates, depreciation, combat activity levels, and the supplies destroyed and captured by ground and air forces during the month. Additional these supply requirements were stratified by source based on what portion could be obtained internally in South Vietnam and what portion must be obtained externally through the borders with North Vietnam, Laos and Cambodia. Admittedly, these values based on a number of assumptions, were rough, but they provide some insight into the North Vietnamese and Vietcong supply requirements—if not in an absolute sense, at least in a relative sense.

Estimated enemy minimum logistics requirements in South Vietnam declined over time from a total of 300 tons per day during Commando Hunt I to about 200 tons per day Commando Hunt VII. This decrease resulted from both declining enemy strength and activity levels and from revisions in basic assumption factors. The average tonnage requirement was 240 tons per day, of which 205 tons, or 85 percent, was food. The bulk of this food, about 80 percent, was obtained within South Vietnam and Cambodia. The remaining tonnage was comprised of equipment, weapons, and ammunition. Automotive fuel requirements, considered to be minimal, were not included.

The estimated minimum requirements from the trail averaged 35 tons per day, or 15 percent of the total. When combined with throughput tonnages from the trail, these estimates provide the stock profile presented in Figure 3. As stated above, caution should be exercised in interpreting the absolute values diagrammed in the figure. The profile depicts the cumulative amount of estimated supplies that flowed through the trail from the beginning of Commando Hunt I minus the estimated enemy minimum requirements from the trail during the same time period. All values are trail-related and exclude internal requirements and acquisitions, flows through Cambodia before the port of Kompong Som was closed in 1970, and the leakages and estimated preinvasion movement of 400-800 tons through the demilitarized zone. If throughput was underestimated or minimum requirements were overestimated, stock levels from the trail would be higher than depicted; if the opposite were true, the level would be lower. There is, then, a degree of uncertainty associated with the height of the stock profile.

However, if any validity can be attached to the profile, several factors become apparent. First, the North Vietnamese broke about even as a result of the resupply effort during Commando Hunt I and, perhaps as a result of this and the prospective loss of the Cambodian LOC, launched a major supply offensive during Commando Hunt III. After that time, however, the stock level trend became unfavorable to the enemy. We might speculate that the increasingly effective interdiction effort influenced his decision to launch the 1972 invasion of South Vietnam before stock levels again approached zero, but the truth may never be known. The enemy rationale that led to the invasion is but another of the many unknowns that contribute to the uncertainty over the impact of the Commando Hunt campaigns.

Second, the profile indicates that the enemy had the logistics capability in March 1972 to launch an offensive in South Vietnam. Certain critics have advanced the argument that the invasion invalidated all previous logistics data because the enemy demonstrated the ability to support an invasion in spite of low throughput predictions. However, the enemy supply requirement from the trail, which contained the weapons and ammunition that could not be obtained elsewhere, was important but not large. It would be simple accumulate a sizable supply stock in light low-activity levels experienced during previous years. Complete interdiction of a flow supplies is impossible, and without force expenditure at the destination, a build-up is inevitable.

This does not mean the enemy was able to position the right supplies at the right place during the ensuing invasion—only that the aggregate tonnage appeared sufficient for an offensive. In fact, estimated throughput from the trail and the demilitarized zone during April and May was 4600 tons, and the estimated minimum requirement from both was 5300 tons. This decrease of 700 ton was only ten percent of the estimated stock level; yet, the enemy offensive had been blunted and was completely contained by the end of May. North Vietnamese objectives, which at a minimum included Hue, Kontum, and An Loc, remained unrealized. From all indications, air power had devastated the enemy’s capability to continue the offensive. ¹¹

This is somewhat reminiscent of Operation Strangle in Italy, where German aggregate supply tonnages were sufficient for continued operations even after the Allied ground offensive, yet the German defensive posture was broken when tactical interdiction strikes completely overwhelmed the distribution system. It was impossible for German commanders to move and position men and materiel to the right place at the right time. Mobility denial, rather than supply denial, had been the key to the Allied success. Supply denial has seldom, if ever, proved to be a viable objective, and the experience in Southeast Asia tends to substantiate the validity of this premise. ¹²

Observations

One of the stated objectives of the interdiction campaigns in Southeast Asia was to make the North Vietnamese pay an increasingly greater cost for aggression in the South. Air interdiction, directed at supply denial, does raise the cost of operations to the enemy; but in a limited war context, this cannot be a primary objective. For one thing, the increasing cost argument often leads to a double standard. While U.S. efforts are considered successful if they impose and increasing cost on the enemy, the increased cost imposed on the U.S. by the enemy’s initial or counter efforts is not included in the game matrix. (Nevertheless, in the end U.S. withdrawal from Southeast Asia was predicated, in part at least, on the high cost of continuing the war.) Furthermore, in the North Vietnamese case, the cost to the enemy of replacing bomb damage in southern Laos was largely shifted through external aid to other nations of the Communist bloc. The cost to North Vietnam was mainly the opportunity cost of resources used along the trail. The supplies, trucks, construction equipment, and trained personnel employed in Laos could not be used to rebuild the North Vietnamese economy which had never fully recovered from the 1965-68 bombing campaigns. The fact that they continued logistics operations in southern Laos, however, indicates that these costs were bearable.

The increasing cost objective might more appropriately be applied to the December 1972 bombing of the North Vietnamese heartland. This campaign was aimed at applying maximum pressure through destruction of major target complexes in the vicinity of Hanoi and Haiphong. The large, concentrated strike effort severely damaged some of North Vietnam’s most important ad costly military and industrial facilities.¹³ These particular facilities, which are of greatest interest if the increasing cost objective is employed, were previously restricted from air attack. As a result of these restrictions, less valuable interdiction targets along the logistics routes were struck. It is doubtful if the value lost associated with these targets could ever make the cost of continued resupply unbearable.

IN SUMMARY, increasing the cost to an enemy is a necessary but not a sufficient requirement for an interdiction effort. The constraints associated with limited war, by their very nature, relegate this objective to secondary importance. In the end we must return to the original and basic questions: What was the impact of air interdiction on the Communist capability to operate at desired combat levels in South Vietnam? From all indications it was positions but within the range of North Vietnamese tolerance. The true impact, of course, is uncertain, but this uncertainty in and by itself militates against the future allocation of air resources to long-term supply interdiction—especially if air resources are limited, as they well may be in light of increasing budget constraints.

Indeed, example of the vital role played by air interdiction in the success of friendly ground forces have been cited in this text—the campaigns of Europe, the first year of operations in Korea, and the 1972 North Vietnamese invasion of South Vietnam—but in each case the interdiction effort could be directly related to a major ground action. The more intense the action, the more vital become the interdiction effort in forestalling replacements for depleted enemy forces.

However, the timeliness of replacements, a factor so critical to success in intense, large-scale confrontations between opposing forces, fades into relative insignificance as an element in protracted war. Protracted war implies time, and given time, temporary structure rise to replace destroyed bridges, by-passes circumnavigate interdicted route segments, and men and materiel are diverted from less essential to more critical functions. Moreover, in protracted conflicts characterized by guerrilla warfare, only a minimum of supplies is required, and since the option to fight or withdraw remains open, neither the volume nor timing of replacements is paramount to ultimate success.

In concluding, then, it should be noted that air interdiction has been a victim of the type of wars waged during the past 25 years, wars that degenerated into protracted periods of relative stagnation. Long-term supply interdiction, the version assigned to cover these static periods, could claim few successes. In fact, it is highly unlikely that any military operation—land, sea, or air—could claim success under such conditions.

Office of the Assistant Secretary of Defense

Notes

1. U.S. Air Force Manual 2-1, Tactical Air Operations—Counter Air, Close Air Support, and Air Interdiction, Department of the Air Force, 2 May 1969.

2. F. M. Sallagar, Operation "Strangle"(Italy, Spring 1944): A Case Study of Tactical Air Interdiction, The Rand Corporation, R-851-PR, February 1972.

3. Ibid., p. vii.

4. Ibid., p. xiii.

5. Basil H. Liddell Hart, The German Generals Talk (New York: Morrow, 1948), pp. 243-44.

6. Gregory A. Carter, Some Historical Notes on Air Interdiction in Korea, The Rand Corporation, P-3452, September 1966.

7. Material in this section was extracted from a series of classified reports prepared annually by the Directorate of Tactical Analysis, Headquarters Seventh Air Force, on the interdiction campaigns in southern Laos. These included Commando Hunt, May 1969: Commando Hunt III, May 1970: Commando Hunt V, May 1971; and Commando Hunt VII, June 1972. The extracted material is unclassified.

8. The gunships were C-130 and C-119 transport aircraft that were modified with sophisticated night detection equipment and 20, 40, and later, 105 millimeter cannons to destroy trucks moving down the trails of Laos. These aircraft were by far the most effective truck-killing systems in the U.S. arsenal.

9. Throughput and input were calculated by intelligence analysts who combined the number of southbound sensor-detected truck movements, aircraft visual truck observations, and road and river watch team obtains along the Laos entry and exit routes. Duplicate counts were then eliminated to obtain an estimate of the actual truckloads of southbound supplies that entered and exited the system. To the input figure was added an estimate of equivalent truckloads of supplies that entered Laos through enemy pipelines and natural waterways.

10. A detailed account of the campaign by this writer, entitled "The Commando Hunt V Interdiction Campaign: A Case Study in Constrained Optimization," will be published in a forthcoming issue of Air University Review.

11. It should be pointed out that this represented the first North Vietnamese offensive m South Vietnam that was conducted in an exclusively, conventional mode—complete with tanks, sophisticated crew-served weapons, and large attack formations. Thus the North Vietnamese were clearly more vulnerable to air strikes than in the past.

12. One reason for this is that troop movement requires greater LOC capacity than does supply movement. For example, the road movement of  a U.S. infantry division normally consumes six to eight times more road capacity than does its daily resupply requirement. If the movement is by rail, the capacity difference is even greater, averaging about 135 to one J. W. Higgins, Military Movements and Supply Lines as Comparative Interdiction Targets, The Rand Corporation, RM-6308-PR, July 1970.

13. A classified evaluation of the bombing results can be found Herman L. Gilster and Robert F. M. Frady, Linebacker II USAF Bombing Survey, Headquarters Pacific Air Forces, April 1973.