World War II By karenzk Period: Jan 1, 1939 to Dec 31, 1945 World War II Sep 1, 1939 U.S. makes announcement U.S. announces its neutrality in the European conflict. May 10, 1940 Winston Churchill becomes Britain's prime minister Dec 9, 1941 China declares war on Japan. Apr 12, 1945 Harry Truman becomes President of the U.S. Vice-President Harry Truman sworn in as President upon the death of President Roosevelt. Aug 9, 1945 U.S.S.R. declares war on Japan.
