Pusan Perimeter offensive
The Pusan Perimeter offensive was a large-scale offensive by United Nations Command forces against North Korean forces commencing on 16 September 1950.
UN forces, having been repeatedly defeated by the advancing North Koreans, were forced back to the "Pusan Perimeter", a defensive line around an area on the southeastern tip of the Korean Peninsula that included the port of Pusan. The UN troops, consisting mostly of forces from South Korea and the United States mounted a last stand around the perimeter in the Battle of the Pusan Perimeter, fighting off repeated North Korean attacks for six weeks as they were engaged around the cities of Taegu, Masan, and Pohang and the Naktong River. The massive North Korean assaults were unsuccessful in forcing the UN forces back further from the perimeter, despite two major pushes in August and September. Following the UN counterattack at Inchon on 15 September, on 16 September UN forces within the Pusan Perimeter mounted an offensive to drive back the North Koreans and link up with the UN forces at Inchon.
Background
The Inchon landing put the US X Corps in the rear of the Korean People's Army. Concurrently, US Eighth Army was to launch a general attack all along its front to fix and hold the KPA's main combat strength and prevent movement of units from the Pusan Perimeter to reinforce the threatened area in his rear. This attack would also strive to break the KPA cordon that had for six weeks held Eighth Army within a shrinking Pusan Perimeter. If Eighth Army succeeded in breaking the cordon it was to drive north to effect a linkup with X Corps in the Seoul area.Planning
The Eighth Army, commanded by General Walton Walker, planning was underway by 6 September, becoming an operational directive on the 16th. The attack was set for 09:00 on 16 September, one day after the Inchon landing. The U.S. Eighth and the ROK armies would attack along the Taegu-Kumch'on-Taejon-Suwon axis, destroy KPA forces encountered, and link up with X Corps. The new US I Corps in the center of the Perimeter line was tasked with making the main breakthrough; its distance to X Corps was the shortest, and the road network available to it were better with easier grades which would benefit mechanized warfare. US 5th Regimental Combat Team and the 1st Cavalry Division would seize a bridgehead over the Naktong River near Waegwan, allowing the US 24th Infantry Division to cross and drive on Kumch'on-Taejon. 1st Cavalry Division would secure the rear and the line of communication. At the same time, US 25th and 2nd Infantry Divisions, in the south on the army left flank, and the ROK II and I Corps, on the east and right flank, would attack to fix the KPA forces opposite them in place and exploit local breakthroughs. The ROK 17th Regiment would be sealifted from Pusan to Inchon to join X Corps.To support seizing the bridgehead at Waegwan, the US 2nd and 24th Divisions would cross the Naktong below Waegwan and the ROK 1st Infantry Division above it. Difficulties were expected because there was insufficient engineers and bridging equipment for several quick crossings; Eighth Army had equipment for only two pontoon treadway bridges across the Naktong. Fifth Air Force, commanded by commander General Stratemeyer, received transfers from the 20th Air Force on Okinawa to compensate for the Marine air squadrons diverted to the Inchon operation. The reinforcements were the 51st Fighter-Interceptor Wing and the 16th and 25th Fighter-Interceptor Squadrons and based at Itazuke Air Base, Japan.
Eighth Army could not concentrate a large force for a breakout because most of its strength was fixed along the Perimeter by the KPA, which held the initiative. The only free unit was the US 24th Infantry Division, which moved piecemeal from the east to the center shortly before the attack. Once started, Eighth Army's situation would change from a precarious defense to an offensive without reinforcement or opportunity to create a striking force. In theater perspective, Eighth Army would make a holding attack while X Corps made the envelopment. A prompt link-up with X Corps along the Taejon-Suwon axis was a prerequisite for cutting off a large force of North Koreans in the southwestern part of the peninsula. Eighth Army anticipated that the Inchon landing would demoralize the North Koreans opposite to it and raise its own morale; the one day delay to the army's attack was intended to exploit this. News of the successful landing reached Eighth Army immediately on the 15th, but took several days to reach KPA forces along the Perimeter.
The prospective corridors of advance for Eighth Army's breakout were the same as those used by the KPA's drive south. The KPA blocked every road leading out of the Perimeter. The axis of the main effort had to use the highway from the Naktong opposite Waegwan to Kumch'on and across the Sobaek Mountains to Taejon. The valley of the Naktong northward to the Sangju area was also usable. The Taegu-Tabu-dong -Sangju road traversed this corridor, with crossings of the Naktong River possible at Sonsan and Naktong-ni. From Sangju the line of advance could turn west toward the Kum River above Taejon or bypass Taejon for a more direct route to the Suwon-Seoul area. Eastward in the mountainous central sector, the ROK would find the best route of advance by way of Andong and Wonju. On the east coast the only option was the coastal road toward Yongdok and Wonsan.
Before the attack, Eighth Army directly controlled its four infantry divisions and other attached ground forces of regimental and brigade size. Part of Far East Command's preparations was reactivating I and IX Corps and adding them to the army. On 2 August, I Corps, commanded by General John B. Coulter, was activated at Fort Bragg. Eleven days later Coulter and a command group arrived in Korea and began preparing for the breakout. The main body of the corps staff arrived in Korea on 6 September. On 10 August, IX Corps, commanded by Major General Frank W. Milburn, was activated at Fort Sheridan. Milburn and a small group of staff officers departed Fort Sheridan on 5 September by air for Korea. The main body of the corps staff, however, did not reach Korea until the end of September and the first part of October. Both corps had once been part of Eighth Army in Japan; the I Corps with the 24th and 25th Divisions with headquarters in Kyoto, and the IX Corps with the 1st Cavalry and the 7th Divisions with headquarters in Sendai.
I Corps became the main breakout force. Walker swapped the corps commanders on 11 September; Milburn took command of I Corps that day at Taegu, while Coulter took command of IX Corps the next day at Miryang. I Corps became operational at 12:00, 13 September, with the US 1st Cavalry Division, the 5th Regimental Combat Team and the ROK 1st Division attached. On 15 and 16 September the 5th Regimental Combat Team and the 24th Division moved to the Taegu area, and by the evening of 16 September I Corps included the British 27th Infantry Brigade, the ROK 1st Division and supporting troops.
IX Corps was not operational during the first week of the offensive. The delay was mainly due to insufficient communications personnel and equipment; the signal battalion intended for IX Corps had been diverted to X Corps. It became operational on 14:00, 23 September, with the transfer of the US 25th and 2nd Infantry Divisions and their supporting units. The corps' operations continued to be hampered by inadequate communications facilities.
Opposing forces
On the eve of offensive US intelligence estimated that Eighth Army faced 101,417 KPA troops in 13 infantry divisions, 1 armored division and 2 armored brigades. The KPA I Corps on the southern half of the front had 6 infantry division with armored support for 47,417 men, and the II Corps on the northern and eastern half of the front had 7 infantry divisions with armored support for 54,000 men. KPA formations were estimated to be at an average of 75 percent strength in troops and equipment. According to the estimate, the KPA's Perimeter forces could maintain its defenses after diverting three divisions to the Seoul area, and that it remained capable of offensive action. US intelligence overestimated KPA capabilities; the KPA's Perimeter forces were inferior to Eighth Army in mid-September according to prisoner of war interrogations and captured documents. The KPA 13th Division had 2,300 men plus 2,000 untrained and unarmed replacements according to Colonel Lee Hak Ku, its chief of staff; US intelligence estimated 8,000 men. The practically destroyed KPA 15th Division had no more than a few hundred scattered and disorganized men; US intelligence estimated 7,000 men. The KPA 5th Division had 5,000 men, not the estimated 6,500, and the KPA 7th Division had about 4,000 men instead of the estimated 7,600. The KPA 1st, 2nd and 3rd Divisions were almost certainly below the 7,000-8,000 men assigned to each in the estimate. KPA losses were exceedingly heavy in the first half of September; there has been no accurate accounting.According to Appleman, KPA strength along the Perimeter in mid-September was about 70,000 men, with no more than 50% of their original heavy weapons and tanks. KPA morale was at its nadir. No more than 30% of the original troops remained; the veterans tried to impose discipline on the recruits, most of whom were from South Korea and had no desire to fight for the North Koreans. It was common for veterans to shoot anyone who showed reluctance to go forward when ordered or who tried to desert. Food was scarce, and undernourishment was the most frequently mentioned cause of low morale by prisoners. Even so, there were few desertions up to this time due to officers executing attempted deserters and the belief that UN forces executed prisoners.
By comparison, the Eighth Army and the ROK Army had 140,000 men in combat units in mid-September. The four US divisions averaged 15,000 men each, plus 9,000 attached ROK recruits. The six ROK divisions averaged about 10,000 men each. The three Corps' headquarters added at least another 10,000 men. The addition of the two army headquarters would result in a total of more than 150,000 men.