SCAP identified the Allied occupation headquarters under MacArthur after Japan’s 1945 surrender.
SCAP stood for Supreme Commander for the Allied Powers, the title used for General Douglas MacArthur and for the occupation organization he directed. The headquarters supervised Japan’s occupation from 1945 until the peace treaty took effect in 1952. In practice, SCAP issued directives to the Japanese government and controlled the major military and political decisions of the occupation.
The organization oversaw demilitarization, war-crimes prosecutions, constitutional reform, land reform, and changes to education and labor policy. Japan retained its government institutions, but they operated under Allied authority and had to comply with SCAP directives.
SCAP is sometimes confused with GHQ, meaning General Headquarters, because the occupation headquarters was commonly called GHQ/SCAP. The acronym is not the name of a separate postwar treaty or a Japanese ministry. MacArthur held the Supreme Commander title until 1951, when General Matthew Ridgway replaced him.