United Nations (UN) forces at Inchon soon recaptured Kimpo airfield. There, U.S. planes began to conduct air strikes on North Korean positions in and around Seoul. UN ground forces then launched a three-pronged attack on the capital. They met heavy North Korean resistance at the perimeter but soon captured the heights overlooking the city. On September 25, 1950, UN forces entered Seoul, and soon declared the city liberated. Even then, house-to-house fighting continued until September 27, when the city was brought under full UN control. On September 29, 1950, UN forces formally turned over the capital to President Syngman Rhee, who reestablished his government there. And by the end of September 1950, with remnants of the decimated North Korean Army retreating in disarray across the 38th parallel, South Korean and UN units gained control of all pre-war South Korean territory.
On October 1, 1950, the South Korean Army crossed the 38th parallel into North Korea along the eastern and central regions; UN forces, however, waited for orders. Four days earlier, on September 27, 1950, President Truman sent a top-secret directive to General MacArthur advising him that UN forces could cross the 38th parallel only if the Soviet Union or China had not sent or did not intend to send forces to North Korea.
Earlier, the Chinese government had stated that UN forces crossing the 38th parallel would place China’s national security at risk, and thus it would be forced to intervene in the war. Chairman Mao Zedong also stated that if U.S. forces invaded North Korea, China must be ready for war with the United States.
At this stage of the Cold War, the United States believed that its biggest threat came from the Soviet Union, and that the Korean War may very well be a Soviet plot to spark an armed conflict between the United States and China. This would force the U.S. military to divert troops and resources to Asia, and leave Western Europe open to a Soviet invasion. But after much deliberation, the Truman administration concluded that China was “bluffing” and would not really intervene in Korea, and that its threats merely were intended to undermine the UN. Furthermore, General MacArthur also later (after UN forces had crossed the 38th parallel) expressed full confidence in the UN (i.e. U.S.) forces’ military superiority – that Chinese forces would face the “greatest slaughter” if they entered the war.
On October 7, 1950, the UNGA adopted Resolution 376 (V) which declared support for the restoration of stability in the Korean Peninsula, a tacit approval for the UN forces to take action in North Korea. Two days later, October 9, UN forces, led by the Eighth U.S. Army, crossed the 38th parallel in the west, with General MacArthur some days earlier demanding the unconditional surrender of the North Korean Army. UN forces met only light resistance during their advance north. On October 15, 1950, Namchonjam fell, followed two days later by Hwangju.
In North Korea’s eastern coast, the U.S. X Corps made unopposed amphibious landings at Wonsan on October 25, 1950 (with South Korean forces having taken this port town days earlier) and at Iwon, further north, on October 29. On October 24, 1950, under the “Thanksgiving Offensive” issued by General MacArthur who wanted the war ended before the start of winter, UN forces made a rapid advance to the Yalu River, which serves as the China-North Korea border.
In late October 1950, UN forces clashed with the Chinese Army, and a new phase of the war began. Earlier, in June 1950 in Beijing, Chairman Mao had declared his intention to intervene in the Korean conflict, which received strong reluctance from Chinese military leaders. But with the support of Premier Zhou Enlai, and General Peng Dehuai, (military commander of China’s northwest region, who would be appointed to lead the Chinese forces in the Korean War), the plan for the Chinese Army (officially called the People’s Liberation Army, or PLA) to become involved in Korea was approved.
On October 8, 1950, the day that UN forces crossed the 38th parallel into North Korea, Chinese forces in Manchuria (the North East Frontier Force, or NEFF) were ordered to deploy at the Yalu River in preparation to enter North Korea from the north. On October 19, 1950, the day Pyongyang fell, on Chairman Mao’s order, the NEFF crossed into North Korea. Chinese authorities called this force the “People’s Volunteer Army”, the “volunteer” designation conferring on it a non-official status in order that China would not be directly involved in a war with the US/UN.
The Chinese deployment from Manchuria to Korea was carried out under strict secrecy, and Chinese troops travelled only at night and remained camouflaged during the day. So successful were the Chinese in using secrecy and concealment that U.S. surveillance planes, even with their full control of the skies, were unable to detect the massive Chinese buildup at the Yalu River. Chinese forces soon entered North Korea.
On October 25, 1950, using surprise and overwhelming numerical force, the Chinese struck at the UN forces (led by the Eighth U.S. Army), which was moving up the western region toward the Yalu River. The Chinese particularly targeted the UN right flank along the Taebaek Mountains, which consisted of South Korean forces. In the ensuing four-day encounter at Onjong (Battle of Onjong), the Chinese severely crippled the South Korean forces and punched a hole in the UN lines. Thousands of Chinese soldiers then poured through the gap and advanced behind UN lines. On November 1, 1950 at Unsan, the Chinese attacked along three points at the UN line at its center, inflicting heavy casualties on the American and South Korean forces. At this point, the U.S. high command ordered the Eighth U.S. Army to retreat south of the Chongchon River.
On November 6, 1950, the Chinese forces also broke contact and withdrew north to the mountains. Unknown to UN forces, the Chinese had over-extended their supply lines, which would be a problem that Chinese forces would face constantly during the war. Furthermore, in this early stage of their involvement in the war, the Chinese relied on weapons supplied by the Soviet Union. Later on, the Chinese would also manufacture their own armaments, and reduce their reliance on foreign imports.
The fighting in the north also saw the first air battles between American and Soviet jet planes, leading to many intense dogfights during the war. Early on, the newly released, powerful Soviet MiG-15 easily outclassed the U.S. first-generation jet planes, the P-80 Shooting Star and the F9F Panther, and posed a serious threat to the U.S. B-29 bombers. But with the arrival of the U.S. F-86 Sabre, parity was achieved in the sky in terms of jet fighter aircraft capability on both sides. Ultimately, U.S. planes would continue to hold nearly full control of the sky for the duration of the war.
The sudden Chinese withdrawal during the Battle of Onjong perplexed the U.S. military high command. Weeks earlier, General MacArthur stated his belief that China had some 100,000-125,000 troops north of the Yalu River, and that if half of this number was sent to Korea, his forces easily could meet this threat. In the ensuing lull (November 6–24, 1950), U.S. surveillance planes detected no significant Chinese military buildup, and sightings of enemy troop strength on the ground seemed to confirm General MacArthur’s estimates. Convinced that China was not intending to fully intervene in Korea, General Macarthur launched the “Home-by-Christmas” Offensive, a cautious two-sector advance toward the Yalu River: UN forces in the western sector led by the Eighth U.S. Army as the main attacking force, and in the eastern sector led by the U.S. X Corps to support the attack and also cut off enemy supply and communication lines.