As part of preparations for Operation Downfall, the invasion of Japan, MacArthur became commander in chief U.S. Army Forces Pacific (AFPAC) in April 1945, assuming command of all Army and Army Air Force units in the Pacific except the Twentieth Air Force. At the same time, Nimitz became commander of all naval forces. Command in the Pacific therefore remained divided. During his planning of the invasion of Japan, MacArthur stressed to the decision-makers in Washington that it was essential to have the Soviet Union enter the war as he argued it was crucial to have the Red Army tie down the Kwantung army in Manchuria. The invasion was pre-empted by the surrender of Japan in August 1945. On 2 September MacArthur accepted the formal Japanese surrender aboard the battleship , thus ending hostilities in World War II. In recognition of his role as a maritime strategist, the U.S. Navy awarded him the Navy Distinguished Service Medal.