What country did Germany invade on September 1, 1939?

Germany invaded Poland on September 1, 1939, beginning the European war that became World War II. The attack followed years of Nazi territorial demands, including claims over the Free City of Danzig and access through the Polish Corridor.

German forces attacked from the west, north, and south using coordinated tanks, aircraft, artillery, and infantry. The campaign is often associated with Blitzkrieg, although that term was not the official name of a single German operational doctrine. Poland resisted fiercely, including at Westerplatte, the Battle of the Bzura, and during the defense of Warsaw.

The invasion was not solely a German operation. Under the secret protocol of the Molotov–Ribbentrop Pact, the Soviet Union invaded eastern Poland on September 17. Poland was subsequently divided between Germany and the Soviet Union, with smaller territorial changes involving Slovakia and Lithuania. Warsaw surrendered on September 28, and organized resistance ended in early October.

Britain and France had guaranteed Poland's independence and declared war on Germany on September 3. That diplomatic response transformed the attack from a regional conflict into the opening of the European theater of World War II. The invasion of Czechoslovakia had occurred earlier, but it was not the event that began the war in 1939.