Douglas MacArthur became Supreme Commander for the Allied Powers during the occupation of Japan.
MacArthur received the appointment on 14 August 1945, as Allied planning shifted from defeating Japan to supervising its surrender and reconstruction. He arrived in Japan later that month and established his headquarters in Tokyo.
As SCAP, MacArthur exercised authority through the existing Japanese government rather than replacing it with a direct Allied administration. His headquarters issued directives covering demilitarization, political reform, civil liberties, education, and economic policy.
MacArthur also oversaw the early occupation's relationship with Emperor Hirohito. The emperor remained on the throne, but his political and military authority was sharply reduced. Japan's postwar constitution, effective in 1947, renounced war and established a parliamentary system.
MacArthur remained SCAP until President Harry Truman relieved him in April 1951. The Allied occupation formally ended when the Treaty of San Francisco came into force in 1952.