During WWII Nimitz was Commander in Chief, United States Pacific Fleet and Commander In Chief, Pacific Ocean Areas with operational control over Allied air, land and sea units. In December 1944, he was promoted to Fleet Admiral, the highest grade in the Navy. Nimitz signed for the United States when Japan formally surrendered on board the battleship Missouri in Tokyo Bay on September 2, 1945. Nimitz was the leading U.S. Navy authority on submarines. He oversaw the conversion of these vessels' propulsion from gasoline to diesel, and then later was key in acquiring approval to build the world's first nuclear-powered submarine.