On September 15, 1950, United Nations forces landed at Inchon on the west coast of Korea, about 40 kilometres from Seoul and more than 200 kilometres behind the North Korean front. The X Corps, built around the 1st Marine Division and the 7th Infantry Division, put ashore at a port that almost everyone involved considered impossible. It worked, and it reversed the Korean War in eleven days.
Inchon was a bad beach in every respect. The tidal range is among the largest in the world, around ten metres, so for much of the day there is mud flat rather than water. The approach is through a narrow, winding, mined channel between islands, and any ship in it can be fired on from the shore. The harbour front is a seawall, so the troops had to scale it with ladders. There were only a few days in each month when the tide was high enough at the right time of day, and September 15 was one of them with the next window a month away.
MacArthur's staff, the Navy and the Marines all said it could not be done and proposed other places. The arguments against were technical and correct, and MacArthur's answer was that the difficulty was the reason to do it: the defenders would reason the same way, which is why the North Korean forces at Inchon were thin. He took the plan to Washington in August. He got approval.
The landing went ahead. Wolmido island was bombarded in the morning, and the main assault came in the late afternoon, on the second high tide. Marine casualties were light. A few hundred, in the first days. Kimpo airfield was taken on September 18. Seoul, which had fallen in June, was recaptured on September 28 after eleven days of house-to-house fighting that cost far more than the landing.
The North Korean army, which had held the UN forces in a shrinking perimeter around Pusan, was cut off from its supplies and disintegrated, with tens of thousands taken prisoner. Within a month UN forces were across the 38th parallel and driving for the Yalu.
That is where the success turned into something else. Then China came in. MacArthur's forces had reached the Chinese border in two thrusts separated by roadless mountains. Chinese forces entered the war in late October and November 1950, and by January had driven them back south of Seoul. The war settled into the line it had started on and ended in an armistice in July 1953.