Who were the presidents during WWII?

President Franklin Roosevelt signed the Declaration of War against Japan, in December 1941 shortly after the attack on Pearl Harbor. He remained president for the duration of war until his death in office in April of 1945, less than a month before the surrender of Germany. President Harry Truman succeeded him and saw the end of the war under his administration.

Franklin Roosevelt became President in 1933. Hitler became Chancellor of Germany in that year. Hitler marched all over his neighbors for some years but the War in Europe properly broke out in 1939. The U.S. entered the War after Japan attacked the U.S. at Pearl Harbor in Hawaii in December of 1941. Roosevelt remained President until his death in April of 1945, at which time his Vice President, Harry S Truman (no dot after the S!) became the President.

Truman oversaw the end of the War in August of 1945.

So there were two, Roosevelt and Truman.