Hitler systematically violated the Treaty of Versailles through a series of military actions and annexations between 1933 and 1939. He remilitarized Germany, occupied the Rhineland in 1936, annexed Austria in 1938, and invaded Czechoslovakia in 1939. In August 1939, Hitler signed a non-aggression pact with Stalin, allowing Germany to invade Poland unopposed, starting World War II. By June 1940, Germany had conquered France and Western Europe, leaving Britain as Hitler's sole remaining opponent. However, Hitler's invasion of the Soviet Union in 1941 opened a deadly new front that ultimately contributed to Germany's defeat.