Douglas MacArthur was appointed Supreme Commander for the Allied Powers to oversee Japan's surrender and occupation.
MacArthur had been a senior U.S. commander in the Pacific and had led the Allied return to the Philippines. In August 1945, President Harry Truman selected him to supervise the surrender process and the occupation of Japan.
MacArthur's headquarters prepared surrender procedures, issued General Order No. 1, and coordinated the initial occupation. He also presided over the 2 September ceremony aboard the USS Missouri in Tokyo Bay.
Chester W. Nimitz was the U.S. representative who signed the Instrument of Surrender after MacArthur. That separate role is why Nimitz is associated with the ceremony but was not the officer appointed as SCAP.