Developments leading to the beginning of World War 2. Lack of enforcement of Treaty of Versailles. Hitler acquiring Austria and parts of Czechoslovakia. Japan Empire invasion of China/Manchuria early 1930's. Japan joining Axis Countries. Germany and Russia invasion of Poland 1939. Russia joins Allied Countries. Japan attack on US Naval ship Pearl Harbor 1941 due to revolt against checks on SE Asia expansion. U. S declares war on Japan 1941 : FDR Administration.