The conflict escalated into warfare when North Korean military (KPA) forces—supported by the Soviet Union and China—crossed the border and advanced into South Korea on 25 June 1950.
The Battle of Osan, the first significant US engagement of the Korean War, involved the 540-soldier Task Force Smith, which was a small forward element of the 24th Infantry Division which had been flown in from Japan. On 5 July 1950, Task Force Smith attacked the KPA at Osan but without weapons capable of destroying the KPA tanks. They were unsuccessful; the result was 180 dead, wounded, or taken prisoner.
Against the rested and re-armed Pusan Perimeter defenders and their reinforcements, the KPA were undermanned and poorly supplied; unlike the UN forces, they lacked naval and air support. To relieve the Pusan Perimeter, General MacArthur recommended an amphibious landing at Incheon, near Seoul and well over 160 km (100 mi) behind the KPA lines. On 6 July, he ordered Major General Hobart R. Gay, commander of the US 1st Cavalry Division, to plan the division's amphibious landing at Incheon.
On 27 August, 67th Fighter Squadron aircraft mistakenly attacked facilities in Chinese territory and the Soviet Union called the UN Security Council's attention to China's complaint about the incident.
On 16 September Eighth Army began its breakout from the Busan Perimeter. Task Force Lynch, 3rd Battalion, 7th Cavalry Regiment, and two 70th Tank Battalion units (Charlie Company and the Intelligence–Reconnaissance Platoon) advanced through 171.2 km (106.4 mi) of KPA territory to join the 7th Infantry Division at Osan on 27 September.
On 25 September, Seoul was recaptured by UN forces. US air raids caused heavy damage to the KPA, destroying most of its tanks and much of its artillery. KPA troops in the south, instead of effectively withdrawing north, rapidly disintegrated, leaving Pyongyang vulnerable.
On 27 September, MacArthur received the top secret National Security Council Memorandum 81/1 from Truman reminding him that operations north of the 38th Parallel were authorized only if "at the time of such operation there was no entry into North Korea by major Soviet or Chinese Communist forces, no announcements of intended entry, nor a threat to counter our operations militarily".
On 30 September, Zhou Enlai warned the US that China was prepared to intervene in Korea if the US crossed the 38th Parallel. Zhou attempted to advise KPA commanders on how to conduct a general withdrawal by using the same tactics that allowed Chinese communist forces to successfully escape Chiang Kai-shek's Encirclement Campaigns in the 1930s, but by some accounts KPA commanders did not use these tactics effectively.
In 19 October 1950, Mao made the decision to send the People's Volunteer Army (PVA), a special unit of the People's Liberation Army, into the Korean war and fight as well as to reinforce the armed forces of North Korea, the Korean People's Army, which had been in full retreat. Historical records showed that Mao directed the PVA campaigns to the minutest details.