On 15 September 1950, after hurling itself fruitlessly against the Pusan Perimeter for nearly a month and a half, the weakened North Korean army was suddenly confronted with a grave threat in its rear. U.S. Marines had landed at the western port city of Inchon, near Seoul, and were poised to move inland to retake the capital and decisively cut the already tenuous North Korean supply lines.

This daring amphibious operation was conceived by General of the Army Douglas MacArthur. Though strategically tempting, Inchon was a tactically challenging amphibious target, with long approaches through shallow channels, poor beaches and a tidal range that restricted landing operations to a few hours a day. It took all of MacArthur's unparalleled powers of persuasion to sell his concept to doubting Army, Navy and Marine Corps commanders.

Forces gathered for the Inchon invasion included the First Marine Division, the Army's Seventh Infantry Division, some South Korean units, virtually every available amphibious ship, and dozens of other Navy warships. Most of the Marines had recently arrived from the U.S., while the rest were withdrawn from the Pusan Perimeter defenses.

Preliminary naval gunfire and air bombardment began on 13 September. The 1st and 5th Marines went ashore on the morning of the 15th. Resistance and casualties were modest, and initial objectives were quickly secured. Over the next several days, as supplies and troops poured ashore at Inchon, the Marines moved relentlessly toward Seoul. Kimpo airfield was taken on 17 September and was in use to support operations two days later. On 29 September, after days of hard street fighting, Seoul was returned to the South Korean government.

A hundred miles to the southeast, the Pusan Perimeter's defenders went on the offensive on 16 September. After resisting for a few days, the now-isolated North Korean army retreated and progressively collapsed during the rest of the month. On the 27th, U.S. Army units moving southwards from Seoul met those coming up from Pusan.