Beginning in July, as UN forces retreated through South Korea, MacArthur readied a seaborne invasion near Seoul to cut off the North Korean forces in the South from their supplies in the North. The area he selected was Inchon. He began the planning phase of the operation despite the opposition of the Joint Chiefs of Staff and other senior Army and Navy officers, the former objecting to the difficult terrain and the latter concerned about the high tides and tricky currents the landings would encounter. In the end, MacArthur persuaded the opposition to go ahead with the landings, arguing that they would cut off the North Koreans around Pusan and avoid the necessity for a winter campaign.
Before the landings American and South Korean infiltrators landed near Inchon to observe the tides and the defenses around the area. US bombers used napalm to burn much of the terrain in the area, to open a path for the landing troops. US and Canadian destroyers shelled North Korean batteries in the area for several days before the landing, sustaining casualties from the return fire. The destroyers also destroyed mines, supplied by the Soviets in the early days of the war, using gunfire.
On September 15, following a naval and air bombardment of the landing areas, the landings at Inchon were met with relatively light resistance. The North Koreans had been led by an American disinformation campaign to expect a landing, but elsewhere, and Inchon was defended by significantly fewer troops than were landed by the UN. The invading force outnumbered the defenders by more than six to one. The following day the North Koreans launched an armored counterattack with Soviet built T-34 tanks, which were destroyed by air and ground attacks.
On September 17 US Marines attacked the airfield at Kimpo, the largest aviation facility in Korea, and captured it relatively intact, giving US Air Force fighter and attack planes an operational base and opening a logistics facility through which supplies and equipment were delivered from Japan. With Inchon secure and an operational air base in UN hands, the drive to recapture Seoul began. It was a hard fought and bloody operation to enter the city, and once US Marines were in Seoul they were involved in difficult, house to house fighting.
Seoul was declared to be under UN control on September 25, 1950, while there were still some pockets of resistance in some areas of the city. As the fighting for Seoul was underway the American and other UN troops in Pusan broke out of the pocket, driving the North Koreans out of South Korea. In the first two weeks of September, 1950, up to 41,000 North Korean troops were killed or captured, and about 30,000 escaped the UN forces to return north of the 38th parallel into North Korea. There they were resupplied with weapons and equipment provided by the Soviet Union. US troops were not authorized to pursue them above the parallel, but the South Korean army was, and did.