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6. Air Interdiction

Clausewitz said a century and a half ago that combat -- battle -- was the essence of war. Under some circumstances, it might not be necessary to engage in actual battle. But even in such cases, the threat of war, if not the actuality, determined victory or defeat.89

Battle, in its simplest terms, is the clash of armed men on a front.

For armed men to clash, for there to be battle, the men, their weapons, ammunition, food, and information must get to the front. If they are already at the front, then sustaining support, such as reinforcements and materiel, must reach them. The totality of men and equipment moves from its source to the front along lines of communication that can range from primitive trails to complex air routes. In the case of materiel, sources reach all the way back to the raw material from which the materiel was made. In addition to the lines of communication that run from the source to the front lines of communication also run laterally along the front. Over these lines move troops shifted from one part of the front to another to meet threats or exploit opportunities.

From the earliest recorded times, commanders have sought to place their forces between the enemy and his base. So serious can such an interposition be that during certain periods, notably in the eighteenth century, this act alone, without any battle taking place, often was enough to induce the interdicted side to make peace. Thus, the history of interdiction is as long, and nearly as important, as the history of battle. The advent of the airplane only added a new dimension to this form of warfare.

Many definitions of interdiction exist. Sometimes it is even broken down into subcategories, such as battlefield interdiction. For simplicity, we will consider any operation designed to allow or inhibit the flow of men or materiel from the source to the front, or laterally behind the front, as interdiction. Additionally, as we did in discussing air superiority, we will not make any distinctions between operations directed at the source and those targeted immediately behind the lines.

Thus, an attack on a train carrying iron ore to the smelter is just as much interdiction as destroying a bridge a mile behind the front. Naturally, the time period required for the effect of either to be felt will vary enormously. Even so, both are interdiction and may fit into the commander's theater air campaign.

With the exception of direct attacks on the source of war materiel, the effectiveness of interdiction is tied closely to either the friendly or enemy ground situation. In general, it is most effective when the enemy is under pressure from hostile action or because his own plans demand mobility. To help in visualizing these situations, we will divide ground action into six categories and examine each in detail.

In Retreat

The most serious predicament with which a ground force must deal is a retreat under enemy pressure. Under such circumstances, the ground force must slow the enemy pursuit as much as possible to make time for establishing new lines, evacuation, or arrival of reinforcements. Interdiction can buy the needed time. Unfortunately, problems that led to the retreat -- especially if it is retreat on a theater scale -- probably included loss of air superiority. Under some unusual conditions, however, air superiority may not have been lost and air interdiction may be possible.

The American retreat from the Yalu River in Korea in the late fall of 1950 is a good example.

After a very successful offensive, reaching to the banks of the Yalu, MacArthur's 8th Army and Xth Corps encountered a massive Chinese counteroffensive. Badly outnumbered -- in some cases by as much as 10-to-1 -- MacArthur ordered a general retreat. General Lin Piao, the Chinese commander, started his pursuit with the objective of destroying the 8th Army as far north as possible. To accomplish this end, he had to abandon his previous practice of marching only at night and camouflaging his army during the day. When he started moving in the day and driving at night with convoy lights on, to develop sufficient speed to catch the retiring American forces, he exposed himself to American air.
The Americans, who still had air superiority, exploited Lin Piao's exposure to the maximum. American intelligence, using aerial reconnaissance and extensive interviews of captured Chinese, estimated that in December alone air attacks killed and wounded more than 30,000 Chinese soldiers -- the equivalent of four to five full divisions. The Chinese were unable to sustain such a high casualty rate and were forced to resume their previous practice of marching at night and hiding during the day.90
MacArthur's armies escaped virtually intact, with total casualties of less than 13,000 killed and wounded.91

Interdiction worked well in this case because the Americans had air superiority and because the Chinese were forced to expose themselves in order to carry out their operations. When we examine the other end of the spectrum, we will see what can happen when a retreating force does not have air superiority and is subject to interdiction.

Static Defense Against An Enemy Offensive

The next most serious situation in which ground forces can find themselves is in static defense against an enemy still on the offensive. The early part of the Korean war provides another good example of what air interdiction can accomplish.

By the first part of July 1950, the surprise North Korean attack of the previous month had pushed South Korean forces and American reinforcements to the far south of the Korean peninsula. There, around the port city of Pusan, the allies succeeded in establishing a defensive perimeter -- but one they feared might break at anytime. Despite the severity of the situation, MacArthur wanted to begin a counteroffensive.
He could not do so, however, until significant forces could be brought from the United States. Important to his counteroffensive plans was maintenance of the Pusan perimeter. He decided that it could only be held if his air arm could keep the North Koreans from massing enough men and supplies for a final effort. He opted to use his air to conduct an intensive interdiction campaign. The campaign succeeded and the perimeter held.92
This interdiction operation took place under air superiority (although at this point in the war, neither side had large air forces available).

Offensive Operations on Both Sides

The next step on the spectrum from worst to best is the situation in which both sides are roughly equal and attempting offensive operations. Under these circumstances, neither side may have sufficient air superiority to conduct an effective interdiction campaign. If an interdiction opportunity does present itself, however, it can pay big dividends. The desert battles of 1941 in North Africa provide an interesting example of the case where neither side had air superiority in the immediate battle zone, but where one side, the British, was able to conduct effective interdiction some distance from the actual fighting.

After initial British successes, under Gen Richard N. O'Connor, fighting in the desert became inconclusive until Rommel assumed command of Axis forces. By the late fall of 1941, Rommel had driven the British back to the Egyptian border and was poised for a final offensive. While suffering reverses in the ground battle, however, the British had waged an intensive interdiction campaign from Malta against Axis shipping to Libya and Tunisia. The campaign reached its height in the fall of 1941, when British air and naval units succeeded in destroying in September 38.5 percent of all supplies sent to Rommel. In November, the British destroyed 77 percent.93
Thus, in December, Rommel was down to 40 tanks, his ammunition stocks were dangerously low, and he was told by the Italians that they had no way to get anything to him for at least another month. He had no option but to retreat from Tobruk and the Egyptian frontier.94

After Rommel's reverses in the desert, the German high command belatedly recognized that British interdiction operations from Malta were intolerable; consequently, they mounted a massive air attack on Malta that came close to causing the island's garrison to surrender. British naval and air units no longer were able to operate from the island. The air attacks, which began in December, had an immediate salutary effect, in January 1942, the Germans lost only 20 percent of their shipping.95 Taking advantage of the neutralization of Malta, they moved sufficient supplies to Rommel to permit him to undertake a major offensive in April.

The interdiction from Malta worked because the British had air superiority over and near it, and because the British in North Africa were maintaining more pressure on Rommel than he could stand. The interdiction effort came to a grinding halt when the Germans seized air superiority over the island. Malta is a classic case illustrating what can happen when things apparently peripheral to the main operation are ignored. Had the Italians taken Malta at the beginning of the war -- as they could have -- or had the Germans mounted their air attacks on it some months earlier -- as they could have -- Rommel quite possibly would have prevailed.

Offensive Operations Against a Static Defense

Let us next examine the condition in which ground forces are intent on launching an offensive against an enemy in static defense. To withstand the attack, the enemy must have sufficient supplies and, except in extraordinary cases, must have the ability to commit reserves and move forces from one point in the line to another. Interdiction can restrict both moves, but experience indicates that its principal benefit is in slowing or stopping the movement of reserves and reinforcements. Two campaigns, the Allied attack on the Gustav Line and the Allied invasion of Normandy, are illustrative.

In the fall of 1943, the Germans in Italy established a fortified defense, the Gustav Line, along the Garigiliano and Rapido rivers on the west and on the Sangro river on the east side of the Italian peninsula. The Allies made attack after attack on the Gustav Line, starting in October 1943. Losses were high and gains were negligible. In an effort to break the German defenses (and minimize casualties), the American air forces began operation "Strangle" in March 1944. Designed to stop the flow of supplies to the Germans, it focused on railroads and roads well north of Rome. In the first week, the Allies cut every railroad in at least two places. Thereafter, they averaged 25 cuts per day. Rail capacity fell from 80,000 tons per day to 4,000, well below what the Germans needed to resist an intensive offense. On 4,000 tons a day, however, the Germans could survive in the absence of Allied ground attack. Thus, they did not withdraw.96
The next step then was resumption of the ground offensive. In preparation for the start of the offensive,

The US air forces continued operation "Strangle," but shifted the focus to the area immediately behind the German lines to just north of Rome. The Allies launched the ground attack in the middle of May, broke through quickly, and in the 14 days after the initial attack, linked up with the beleaguered beachhead at Anzio. In another 10 days, they took Rome.
In just over three weeks, the Allies, who in the preceding six months had achieved nothing with great losses, now moved 80 miles and forced the Germans into precipitate retreat. Losses were relatively light. Since the ground force ratios did not vary significantly from what they had been during the abortive attacks of the late fall and early winter, air interdiction apparently had done the job -- as indeed it had, but not quite in the way that had been anticipated.

Postwar interviews with German commanders, and reviews of German records, indicated that the Germans had sufficient supplies on hand to meet the attack. The northern interdiction had not done too much damage from that standpoint. What made an enormous difference, however, was the German inability to move reserves to the front or to move forces laterally along it. The interdiction campaign had taken such a toll of trucks and trains, and had done so much damage to bridges, railroads, and roads, that the Germans were dependent on foot power and animal transport to move anywhere. Interference before and during the offensive with lateral lines of communication was especially effective.97

General Frido von Senger und Etterlin, commander of the XIV Panzer Corps during the battle, said that enemy air control created difficulties for the German defenders to move troops laterally as was required. He was only able to move at night. He noted that the commander who could only move during darkness was like a chess player allowed only one move for each three made by his opponent.98

The interdiction campaign in Italy succeeded because the Allies had air superiority and were able to keep constant pressure on the lines of communication. It did not, however, force the Germans to retreat. A ground offensive was needed to do that. The offensive succeeded where previously it had failed because the interdiction effort kept the Germans from moving forces needed to plug gaps in the line.

The concept of combining an interdiction campaign with an offensive on the ground is of such importance as to merit another example. The Allied invasion of Normandy was planned in the full knowledge that German forces in northern France would greatly outnumber the invaders. The only way the invasion could succeed was by preventing the movement of reinforcements to the Normandy area. The planners depended on an interdiction campaign to accomplish that end.

The Germans had two basic options for defense against an invasion they knew would come.

First, they could put everything they had on the beaches and hope they would be either sufficiently strong everywhere or that they would correctly anticipate the site of the landings. Under this option, the Germans hoped that the Allies would never be able to establish a beachhead.

Second, in consonance with German doctrine, they could keep forces in reserve until the main landing was clear. They then could hurl superior forces against the beachhead to destroy it.

Rommel, who had experience trying to move forces when the enemy controlled the air, argued strongly for the first option. von Rundstedt, who had no significant experience with enemy air, argued just as strongly for the second.

Von Rundstedt won.99

The numbers alone would seem to have justified von Rundstedt's position. Under his command, he had a million-and-a-half men organized in 60 divisions.100 Contrasted to von Rundstedt, the Allies were only able to put ashore a total of 325,000 men in the first week.101

The difference was in air power. The Allies had conducted a two-month interdiction campaign before the invasion. On D-day, the Allies flew 14,000 sorties, opposed to 100 the Germans managed to put in the air. (Allied air losses from all causes were 127 aircraft, while German losses were 39.)102

The interdiction campaign had two phases.

The first phase, begun in the early spring of 1944, was designed to overwhelm the German transport system by destroying railroads, bridges, and rolling stock.103

The second phase was to prevent the movement of German forces to Normandy after the invasion started.

The success of the first phase is evidenced by a report from Colonel Hoffner, officer in charge of railroads in von Rundstedt's area.

In May 1944, he told von Rundstedt that the Germans needed 100 trains a day. The Germans, however, only managed to average 32 trains a day in that same month -- down from 60 a day in April. May traffic was only 13 percent of what it had been in January.104

The interdiction campaign had crippled the Germans in France before the Allies waded ashore at Normandy. Reports of senior German generals attest to the success of the effort; von Rundstedt said, "the Allied Air Force paralyzed all movement by day, and made it very difficult even at night." von Kluge, von Rundstedt's successor, communicated, "the enemy's command of the air restricts all movement in terms of both space and time, and renders calculation of time impossible." and Rommel commented, "Our operations in Normandy are tremendously hampered, and in some places even rendered impossible" and "the movement of our troops on the battlefield is almost completely paralysed."105

The interdiction campaigns in Normandy and Italy were successful. In the first case, they allowed a landing to succeed that was impossible without them. In the second, they permitted an offensive to succeed when it previously had failed with heavy loss. Two things stand out about these operations: the interdictors had complete air superiority and the defenders were put under enormous pressure by the attackers. Together, the two are powerfully synergistic.

Against a Retreating Enemy

Napoleon once observed that no sight is dearer to the soldier than the knapsack of his enemy. Indeed, that sight is even dearer to the modern soldier who has air power to help him in the pursuit. The retreating army is especially vulnerable to interdiction for several reasons.

Taken together, these four factors make a retreating army an ideal target for air action.

The concept of air interdiction against an army in retreat should be clear enough that it does not need many supporting examples. One should be adequate.

After the Allies broke through the Gustav Line in Italy in May 1944, the Germans fell into a rapid retreat. A retreating army is not necessarily a routed army, and the Germans maintained fair order as they pulled back to the north. Despite their relative order, they still lost more than 70,000 men killed and wounded and a great amount of equipment to air action.106
Of course, the preceding interdiction campaign, which had made movement to the south difficult, also made retreat to the north difficult. Compare these losses to the losses suffered by the Americans when they retreated from the Yalu River in Korea. The difference comes from the fact that the Germans had lost air superiority, while the Americans had maintained it.

Against Self-Sufficient Forces

The last category in the spectrum is the special situation where the enemy force is self sustaining, or nearly so. Guerrillas sometimes are nearly self sustaining in early phases of their war. Even main force units can be almost independent when they are fighting a very low-intensity war, or if they are under no great pressure from their enemy. Obviously, a force that needs little or nothing to exist or fight does not need the kind of supply lines that make air interdiction worthwhile.

Examples are copious and include the Vietnam War before North Vietnamese army forces moved to the south in strength around 1965, the Mau Mau uprising in Kenya, and partisan operations in the Balkans in World War II. The mere fact that a thing called air interdiction exists does not mean that it is appropriate for all conflicts. In the examples cited, it wasn't, and any attempt to use it probably represented effort that should have been devoted to something more productive.

We have seen what interdiction can do and where it is most effective -- when the pressure is on the enemy and he needs to move major forces and equipment quickly, such as during a retreat, a pursuit, or a defense against a determined offense.

Now, we must look at where to interdict. A simple three-level categorization based on relative distance from the front gives us a framework for analysis and planning. Interdiction can be close, intermediate, or distant.

We may define distant interdiction as an attack against the source of men and materiel, or, in the case of a warring party that has no industry, the ports or airfields where materiel provided from outside enters the country.

Intermediate interdiction occurs somewhere between the source and the front.

Close interdiction is interdiction in that area along the front where lateral movement takes place.

It is possible to concentrate on one, or on all together if the circumstances are proper and enough air resources are available for proper concentration on each area. Again, examples from past conflicts will give the reader an idea of the problems and opportunities associated with each.

Distant interdiction has the capability of producing the most decisive outcomes affecting the whole theater -- or even theaters -- but it also has attached to it the greatest time lags between attack and discernible result at the front. For instance, if every oil refinery in the world blew up tomorrow, oil-based industry and transportation wouldn't be forced to shut down the following day. In some cases, they could continue to operate for weeks or even months. Eventually, though, they would stop if the refineries were not rebuilt. The lesson for the air and theater commander is that a delay always exists between cause and effect. If the commander is sure that the war will be decided before an effect can be felt from a given action, then it is pointless to waste resources on carrying it out. He needs to be very careful in this assessment, however, for wars are inevitably much longer or much shorter than anyone expects.

The classic examples of distant interdiction are the operations conducted against Japan and Germany in World War II. Since these operations already have been discussed in some detail, it should suffice to recall that four months of bombing attacks against German petroleum facilities reduced aviation gas production by 95 percent. By the end of the war, German tanks, if they were lucky, got only enough gas for a single attack.107

A modern society, let alone a modern army and air force, cannot operate without fuel. After a success of the magnitude discussed above, one need only wait (while maintaining a degree of pressure) for the enemy to collapse. Barring utter fanaticism, which was unexpectedly stronger in Germany than in Japan, the enemy will quit.

Intermediate interdiction also has a time lag associated with it, but one that probably will be less than that for distant interdiction. It has been most suitable in preparing for future operations. For example, the attacks on rail lines in Italy and France did not pay off until the offensive began against the Gustav Line and until the invasion fleet landed at Normandy. Then the payoff was great.

A similar situation existed in the Southwest Pacific, when the American air force caught a huge Japanese convoy moving troops and supplies from Rabaul to the Huon Peninsula. The convoy was destroyed and a whole division lost.108 Its loss did not have an immediate effect on the battle that was taking place a couple of hundred miles to the east. But its effects were felt months later, when MacArthur moved against the Huon bases that the convoy had intended to build up. MacArthur also conducted a continuous campaign against barge traffic that tried to run from Hollandia and Wewak to intermediate locations the Japanese thought would be strong points that would have to be reduced before the Americans advanced. Although MacArthur bypassed most of these bases, the long and effective interdiction campaign had succeeded in isolating them from resupply of food, spare parts, and medicine.109

Thus, the bases he did attack were considerably weakened and were not able to put up the resistance they otherwise might have.

The last of the categories of interdiction -- close interdiction -- seems most useful when a battle is in progress. We have seen how it played such an important role just prior to and during the May 1944 attacks on the Gustav Line, and even more so at Normandy. Another, more recent example occurred during the 1973 Arab-Israeli war.

On Sunday, 7 October 1973, the Syrians committed their armor reserves on the Golan Heights. Three hundred tanks, commanded by the Syrian President's brother, drove to within five miles of the Benot Yacov bridge. Nothing stood between them and the opportunity to debouch onto the plains below Golan except a handful of reservists just arriving at the front. But, just as a serious setback to the Israelis seemed imminent, the Syrian advance "ran out of steam." As it turned out, the Syrians had run out of gas and ammunition because "the Israeli Air Force had destroyed it." The previous night, the Israelis had conducted interdiction operations just behind the front against the convoys of Syrian trucks carrying ammunition and fuel.
This operation, conducted in lieu of close air support despite the desperate ground situation, had a major impact on the battle.110

Interdiction is a powerful tool in the hands of the joint and air commander, a tool he can use either as part of a potentially war winning campaign -- distant interdiction against the source -- or as part of the ground campaign. It will be effective to some degree in almost all situations. But when it is used in support of ground operations, it is most effective when the enemy is under, or is about to be put under, severe pressure. However, some potential problems must be considered.

All of the successful interdiction campaigns we have discussed have been sustained, concentrated efforts. (The Israeli interdiction of Syrian supplies on the Golan was short, but compared to the total length of the war, it probably wasn't much different from the operations in Italy prior to the attack on the Gustav Line.) It is futile to believe that one or two missions by a handful of planes are going to accomplish anything lasting. Like everything else we have discussed, mass and concentration are essential.

Interdiction operations are going to lead to loss of aircraft and flyers; thus, it is necessary to ensure that something useful is gained for the loss. One modern aircraft and a highly trained pilot probably are too high a price to pay for one old truck, loaded with rice, driven by a private.

Interdiction operations should not be done at the expense of something more important. That something more important almost certainly will be air superiority. A ground commander will demand interdiction in many instances before air superiority has been won. Interdiction missions, except under unusual circumstances, when the benefit clearly outweighs the risk, should not be attempted in the absence of air superiority. A commander does so at his peril, for he is likely to jeopardize his chances of ever winning. We have seen very clearly what fighting without air superiority is like.

A possible compromise, however, may be between the demand for interdiction in support of ground operations and the need to achieve air superiority as quickly as possible. Simply, some targets may support both. A prime example is fuel.

The same fuel probably will not be used for an enemy fighter and an enemy tank, but it almost certainly will be transported and managed by the same net. Attacks on petroleum consequently will serve both ends. Attacks on the transportation net may do the same. Additionally, the enemy must respond with fighters to strike against his petroleum or transport net. When they do respond, fighters escorting interdiction aircraft can attack and destroy enemy aircraft in the air -- keeping in mind that the most expensive place to take out enemy air is in the air.

Nevertheless, if the previously discussed precepts of mass and numerical superiority are observed, these operations can be doubly profitable.

With this observation, we conclude our discussion of interdiction. The next chapter will deal with the use of aircraft to strike enemy troops directly, at the front, in coordination with friendly ground forces.