Hitler invaded Poland on September 1, 1939 using his new Blitzkrieg tactic. In May 1940, Germany attacked France and drove the British and French armies back to the beaches of Dunkirk where over 300,000 troops were evacuated. Germany then controlled three-fifths of France. From August to September 1940, Germany launched an air offensive against Britain known as the Battle of Britain. On June 22, 1941, Hitler violated the Nazi-Soviet Non-Aggression Pact and invaded the Soviet Union in Operation Barbarossa, advancing quickly until his armies were stopped by winter and Soviet resistance outside Moscow in December 1941.