Walton Walker backed South Korea, and China allied with the Soviet Union in what was to become the first military action of the Cold War. Shortly after 75,000 North Korean troops with tanks invaded South Korea on June 25, 1950, American air and sea forces were ordered by President Harry S. Truman to give South Korean troops support. The U.S. Eighth Army was ordered to intervene and drive the invaders back across the 38th parallel, the border between the two countries. With only four lightly equipped and poorly trained divisions, Walker began landing troops on the southeast side of the Korean peninsula
United Nations Command members providing military forces and other assistance to South Korea "make such forces and other assistance available to a unified command under the United States of America". President Syngman Rhee of the Republic of Korea assigned operational command of ROK ground, sea, and air forces to General MacArthur as Commander-in-Chief UN Command (CINCUNC) in a letter (the "Pusan Letter") of July 15, 1950: On August 29, 1950, the British Commonwealth's 27th Infantry Brigade arrived at Busan to join UNC ground forces, which until then included only ROK and U.S. forces. The 27th Brigade moved into the Naktong River line west