World War II is the name of the most horrific and catastrophic conflict ever in the history of human civilization. The war (September 1, 1939 - September 2, 1945) spread to Europe , Asia and Africa under the Nazi invasion of Germany . On September 1, 1939, Germany invaded Poland. This marked the beginning of World War II. Franklin D. was president of the United States at the time. Roosevelt. During his presidency, April 12, 1945, Franklin D. When Roosevelt died, Harry S. took over as president. Truman. On September 2, 1945, World War II officially ended.