Who was the president of america when world war two started?

At the point in time when World War II, or the Second World War, the global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis did the intransitive action begin (commence or get going), who satisfies: X is the president (head of a nation state) of The United States of America (also called the United States, the U.S., the USA, America, and the States), the federal constitutional republic comprising fifty states and a federal district?