Korean War

In 1945, as part of the surrender of Japan, the United States agreed with the Soviet Union to divide the Korean peninsula into two occupation zones at the 38th parallel north. This resulted in the creation of two states: the western-aligned Republic of Korea (ROK) (usually referred to as South Korea), and the Soviet-aligned and Communist Democratic People's Republic of Korea (DPRK) (usually referred to as North Korea). After the surprise attack by the DPRK on June 25, 1950 started the Korean War, the United Nations Security Council authorized a United Nations (UN) force to help South Korea. MacArthur, as US theater commander, became commander of the UN forces. In September, despite lingering concerns from superiors, MacArthur's army and marine troops made a daring and successful combined amphibious landing at Incheon, deep behind North Korean lines. Launched with naval and close air support, the daring landing outflanked the North Koreans, forcing them to retreat northward in disarray. UN forces pursued the DPRK forces, eventually approaching the Yalu River border with China. MacArthur boasted: "The war is over. The Chinese are not coming... The Third Division will be back in Fort Benning for Christmas dinner."
With the DPRK forces largely destroyed, troops of the Chinese People's Liberation Army (PLA) quietly crossed the Yalu River. Chinese foreign minister Zhou Enlai issued warnings via India's foreign minister, Krishna Menon, that an advance to the Yalu would force China into the war. When questioned about this threat by President Truman and Secretary of State Dean Acheson, MacArthur dismissed it completely. MacArthur's staff ignored battlefield evidence that PLA troops had entered North Korea in strength. The Chinese moved through the snowy hills, struck hard, and routed the UN forces, forcing them on a long retreat. Calling the Chinese attack the beginning of "an entirely new war," MacArthur repeatedly requested authorization to strike Chinese bases in Manchuria, inside China. Truman was concerned that such actions would draw the Soviet Union into the conflict and risk nuclear war.