Air Force light bombers laid waste to this cluster of storage warehouses west of Pyonggang.
A 3rd Bombardment Group (Light) B-26 Invader conducts a rocket attack on the rail yard at Iri, South Korea, in early September 1950.
The Chinese offensives of April and May 1951 plainly showed that the effects of interdiction had been less than desired. Much evidence, to be sure, pointed to logistical constriction caused by interdiction at a time of greatly increased consumption as an important reason for the failure of the offensive. In particular, intercepted communications revealed that the conveyance of food had suffered with the need to rush ammunition to the front. But the fact remained that the Chinese had amassed enough supplies to hurl six armies (eighteen divisions) against the United Nations Command. This was reason enough to reassess the interdiction program, for which General Stratemeyer had made the Fifth Air Force primarily responsible. An equally compelling reason was that the losses during the program of intensive armed reconnaissance had exceeded the rate at which aircraft could be replaced. In January 1951, Fifth Air Force lost nineteen aircraft of the types used for interdiction (B-26s, F-51s, F-80s, and F-84s) to operational causes; in April the loss climbed to forty-four.
Toward the end of May, General Timberlake launched Operation STRANGLE. It appears that this flamboyant (if unoriginal) name was chosen to impress the ground commanders, who after the major Chinese offensive were perhaps less than convinced of the efficacy of the interdiction to which the Air Force had devoted so much effort. The purpose of STRANGLE was to ease the advance of the Eighth Army to the 38th Parallel and, over the longer term, to prevent the Chinese from resupplying sufficiently to permit their returning to the offensive. STRANGLE’S interdiction zone was slightly deeper than that of the preceding operation: It ran from the enemy’s railheads, which tended to be along the 39th Parallel, south to the front, a distance of about seventy rather than fifty miles. Planners again divided the zone into three sectors, although the Fifth Air Force now patrolled only one, the others having been assigned to the 1st Marine Air Wing and the Navy’s Task Force 77. STRANGLE deemphasized armed reconnaissance. The focus was now not on vehicles, increasingly well protected in their flak-guarded complexes of bunkers, but on roads. All bridges in the zone of interdiction were attacked as a matter of course, but STRANGLE’ distinctive tactic was the cratering of roads and their mining by delayed-action bombs. Historically, these had not been among the more effective techniques for disrupting motor transport because of the ease with which vehicles could avoid such obstacles. But summer was the rainy season in Korea, and as virtually all roads ran along paddy dikes for some portion of their length, the tactic seemed promising. In general, the jet fighter-bombers (F-80s or the increasingly common F-84s) dealt with the bridges, while the roads fell to the F-51s. B-26s dropped cluster bombs around craters to delay repairs. The weapons broke apart in the air, dispensing butterfly-shaped bomblets that fluttered to the earth, where they lay until contact with man or vehicle set them 0ff.
A second squadron of B-26s began night operations in June, quite possibly in part because the United Nations Command had the month before raised by 50 percent its estimate of the trucks available to the Chinese. The night intruders continued to face major obstacles to their effectiveness. In addition to the inadequacies of the B-26, there was no means but the unaided human eye for detecting convoys. The illumination of targets proved scarcely less vexatious. The previous fall’s efforts to use B-29s to drop flares for the Invaders had not worked well: The only flare available for use with the Superfortresses had a failure rate of more than 50 percent. In January Fifth Air Force had begun to use C-47 cargo planes to drop reliable parachute flares obtained from the Navy. The new technique had worked very well, so well that the ground forces had appropriated most of the sorties of the flare-dropping C-47s to support their own operations. A further limitation was that the C-47s could be flown no more than twenty or thirty miles north of the battleline because of their vulnerability to flak and the enemy’s few night fighters. (About April the Fifth Air Force concluded that the Chinese had begun to make limited use of a Soviet version of the Messerschmitt Me 262, as a night interceptor.) Now the C-47s were unable to provide illumination for many intruder missions in the zone of interdiction, let alone for those penetrating nearly to the Yalu. On deeper missions, therefore, B-26s had to provide their own illumination, a practice employed with varying degrees of success since the early days of the war. In February, underwing flares were fitted experimentally to the Invaders. The attempt seems not to have been successful, for in July both wings of B-26s modified a number of aircraft to carry flares in their bomb bays. In a common method of attack, a flare-carrying B-26 accompanied a strafer. Upon detecting a convoy, the latter would block its path with an incendiary bomb; the illuminating aircraft would then prepare the target for strafing by dropping its flares in a line parallel to the road. Single intruders employed a tactic more difficult to accomplish successfully: They dropped their flares and then descended to make figure-eight passes on the target.
Whatever the method, the use of flares to hunt enemy trucking was of limited utility and very dangerous. The least degree of fog or haziness diffused the glow of the flares and blinded the crews. If the flares were dropped too high, or if the aircraft descended below them, the danger from antiaircraft fire increased markedly. Amid the rugged hills of eastern Korea, the intruders could make their firing runs on convoys from altitudes no lower than 5,000 to 6,000 feet. In the west, where the terrain was less forbidding, the passes were made at 1,500 to 2,000 feet. The common opinion within the Fifth Air Force was that even the lower altitude was too high for effective gunnery.
The Communists soon adopted countermeasures. The North Koreans had the responsibility for maintaining the supply routes. They assigned twelve engineer regiments to the task, each of which had three battalions of 550 men. They stationed repair crews strategically along the main supply routes; often as little as three kilometers separated them. Local civilians, additionally, were impressed as needed. The crews usually managed to fill craters in a matter of a few hours; they cleared butterfly bombs by the simple expedients of detonating them with rifle fire or dragging a rope across the stretch of road affected. American aircraft saw a steadily increasing number of vehicles on North Korea’s roads. In early July the G-2 Section of the United Nations Command estimated that the enemy was stockpiling supplies at the rate of 800 tons a day, which promised to give him very shortly an unprecedented degree of logistical preparedness. Fears of a renewed Chinese offensive were acute by September. It was “increasingly apparent,” the Fifth Air Force’s historian noted, “that the intensive aerial effort against the Communists’ highway main supply routes was not proving too successful.” The only evidence of success, in fact, was the mounting claims of the night intruders. In June the two wings of night flying B-26s claimed 554 vehicles destroyed (out of a total of 827 for the entire Fifth Air Force); the next month the supposed total was 711.
The evidence that STRANGLE had failed led the Eighth Army and Fifth Air Force to undertake a joint reappraisal of the Chinese Army’s logistical system. The study was not published until September, but information collected for it from captured documents, the interrogation of prisoners, and various other intelligence sources influenced the planning for the operation that succeeded STRANGLE the Rail Interdiction Program. There emerged from the data a picture of a system of supply which, while primitive in some respects, was on the whole effective. The enemy’s requirements were minimal. South of Sariwon the Chinese had about 600,000 men in sixty divisions. Each division required about forty tons of supplies a day to sustain itself under prevailing conditions. (This estimate was raised to fifty to sixty tons later in the year.) An American infantry division, slightly less than twice the size of its Chinese counterpart, needed 500 tons daily. A high degree of redundancy characterized the entire Chinese logistical system, both in its general organization and in such details as the practice of routinely building several bypasses for even undestroyed bridges.
The Chinese observed the so-called delivery forward principle used by the Soviets in World War II, whereby supplies flowed through a hierarchically organized chain of logistical units, each of which was responsible for supplying the next lower unit. There were six logistical commands, each commanded by a general officer, to support the front in Korea. Below the logistical commands, a system of fours governed: Each logistical command had up to four main depots, each of which served up to four sub-depots. Each sub-depot in turn controlled up to four divisional supply points. This organization was inherently redundant because the Chinese tactical system was triangular: Each army had three divisions; each division, three regiments; and so on. Each subdepot supported an army, with the result that there were four divisional supply points for every three divisions. The extra supply point provided a margin of safety to compensate for losses or to provide for divisions in transit, without prejudicing the supply of those in place. There was a further margin in that each supply point could support 13,000 men for ten days, while each division had only 10,000 men.
Each logistical command controlled a supply base and a transportation section of four motor-transport regiments (each with 120 GAZ trucks), a porter battalion, and an aircraft spotter unit of 1,200 men. These soldiers deployed nightly along the supply routes at intervals of several thousand feet to scout for damage and to provide warning of the night intruders by firing their weapons. The main depots controlled two motor-transport companies, each with sixty-five trucks. Responsibility for the movements of supplies forward of the divisional supply points fell to the divisions themselves. Lacking organic motor transport, they had to rely on porters and oxcarts to convey their requirements to the front. Artillery and mortar ammunition, which the logistical commands conveyed all the way to the front, were the only exceptions.
The joint study concluded that the enemy’s logistical system, while impressive, had its weaknesses. Command and control were hampered by poor communications and a certain rigidity, and “a fatal lack of coordination between the field and logistical commands” was often apparent during offensives. The “main weakness,” however, stemmed “from the use of old and often insufficient vehicles and rolling stock.” These conclusions, following on STRANGLE failure to destroy the enemy’s road net and Bomber Command’s reduced effort against northern bridges, suggested to the Fifth Air Force’s planners that they might profitably attack the railroads with their fighter-bombers. The destruction of bridges alone would not suffice; the Navy had done an effective job of destroying bridges along the eastern coast, only to observe the enemy shuttling trains between the downed spans. An experiment that had been conducted in late July suggested what might be done. For several weeks one wing of the Fifth Air Force had applied STRANGLE technique of cratering roads to rail lines. The latter, being narrower, were more difficult targets. But they were also harder to repair and, most important, none of the aircraft that attacked the railroads had been lost to flak, perhaps because so much of it was covering the main supply routes.
The premise of the Rail Interdiction Program was that if railroad traffic throughout North Korea could be reduced to naught, the Chinese would be unable to supply their front with motor transport alone. The plan was explained to General Vandenberg in considerable detail when he visited Korea in mid-November. According to American estimates, the sixty divisions south of Sariwon required about 2,400 tons of supplies daily. Upon the assumption that the round trip between the forward units and Antung, the logistical center in Manchuria from which the Chinese armies were supplied, took five days, the enemy needed 6,000 trucks to maintain his forces in the battle area, if each GAZ truck carried an average of two tons. Ordnance officers of Eighth Army supplied estimates, based on their own experience, of the probable life-span of these vehicles. They indicated that the attrition rate for 6,000 vehicles under Korean conditions, exclusive of combat losses, was 120 per day. The Fifth Air Force believed itself capable of destroying at least 150 trucks a day. It therefore seemed “conservative” to conclude that the enemy would then lose 250 trucks daily, or 7,500 monthly. “We don’t feel,” Fifth Air Force’s briefer told Vandenberg, “that the Chinese can support a requirement of 7,500 trucks a month and don’t feel that the Soviets would feed them that many.”