On what date did Germany invade Poland, starting World War II in Europe?

Germany invaded Poland on September 1, 1939, beginning World War II in Europe. German forces attacked from the west, north, and south, while Slovak forces joined the operation from the south.

The invasion followed months of escalating pressure over territory and political demands, including Germany's claim to the Free City of Danzig and access through the Polish Corridor. Nazi Germany also staged the Gleiwitz incident, a false-flag attack used as propaganda to portray Poland as the aggressor. At about 4:45 a.m., the German battleship Schleswig-Holstein opened fire on Polish positions at Westerplatte.

Britain and France declared war on Germany on September 3, but their military assistance could not prevent Poland's defeat. The Soviet Union invaded eastern Poland on September 17 under the secret protocol of the Molotov–Ribbentrop Pact, which divided parts of Eastern Europe into German and Soviet spheres of influence. The campaign ended in October, after Germany and the Soviet Union occupied and partitioned Polish territory. September 3 is therefore not the invasion date; it is the date Britain and France declared war.