Germany invaded Poland on September 1, 1939, starting World War II in Europe.
The attack, known as the Invasion of Poland or September Campaign, came from German territory, East Prussia, and German-allied Slovakia. German forces attacked without a formal declaration of war, including an early assault on the Polish outpost at Westerplatte.
Nazi Germany used a coordinated campaign involving aircraft, tanks, artillery, and rapidly advancing ground forces. Britain and France declared war on Germany on September 3 after it refused to withdraw, turning the German-Polish conflict into a wider European war.
A common mix-up is overlooking the Soviet invasion from the east on September 17. Germany and the Soviet Union had secretly agreed to divide Poland, and the campaign ended with Poland occupied by both powers in October 1939.