Germany prepares for invasion of Poland - Aug 31, 1939 - HISTORY.com

At noon, despite threats of British and French intervention, Nazi leader Adolf Hitler signs an order to attack Poland, and German forces move to the frontier. That evening, Nazi S.S. troops wearing Polish uniforms staged a phony invasion of Germany, damaging several minor installations on the German side of the border. They also left behind a handful of dead German prisoners in Polish uniforms to serve as further evidence of the alleged Polish attack, which Nazi propagandists publicized as an unforgivable act of aggression.

At dawn the next morning, 58 German army divisions invaded Poland all across the 1,750-mile frontier. Hitler expected appeasement from Britain and France–the same nations that had given Czechoslovakia away to German conquest in 1938 with their signing of the Munich Pact. However, neither country would allow Hitler’s new violation of Europe’s borders, and Germany was presented with an ultimatum: Withdraw by September 3 or face war with the Western democracies.

At 11:15 a.m. on September 3, a few minutes after the expiration of the British ultimatum, Prime Minister Neville Chamberlain appeared on national radio to announce solemnly that Britain was at war with Germany. Australia, New Zealand, and India immediately followed suit. Later that afternoon, the French ultimatum expired, and at 5:00 p.m. France declared war on Germany. The European phase of World War II began.