By 15 September, the amphibious assault force faced few KPA defenders at Incheon: military intelligence, psychological warfare, guerrilla reconnaissance, and protracted bombardment facilitated a relatively light battle. However, the bombardment destroyed most of the city of Incheon.
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, Stalin convened an emergency session of the Politburo, in which he condemned the incompetence of the KPA command and held Soviet military advisers responsible for the defeat.
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.