The Korean War (1950–1953) saw the hastily formed Provisional Marine Brigade holding the defensive line at the Pusan Perimeter. To execute a flanking maneuver, General Douglas MacArthur called on United Nations forces, including U.S. Marines, to make an amphibious landing at Inchon. The successful landing resulted in the collapse of North Korean lines and the pursuit of North Korean forces north near the Yalu River until the entrance of the People's Republic of China into the war. Chinese troops surrounded, surprised, and overwhelmed the overextended and outnumbered American forces. The U.S. Army's X Corps, which included the 1st Marine Division and the Army's 7th Infantry Division regrouped and inflicted heavy casualties during their fighting withdrawal to the coast, known as the Battle of Chosin Reservoir.