On what day did Germany invade Poland?

The story behind the answer

September 1, 1939 is correct because Germany launched its Blitzkrieg against Poland that morning, following a staged “Gleiwitz incident” used as a pretext. Two days later, Britain and France declared war on Germany, marking the official start of World War II in Europe; the Soviet Union invaded Poland from the east on September 17.

A quick memory aid: 9/1/39 opens WWII in Europe; 6/22/41 is Operation Barbarossa (Germany invades the USSR); 12/7/41 is Pearl Harbor (the U.S. enters the war); 7/7/37 is the Marco Polo Bridge Incident (wider Sino-Japanese War). Linking these dates to their theaters—Europe (Poland), East (USSR), Pacific (Pearl Harbor), and Asia (China)—helps keep them straight.