What border did German troops cross into that started ww2?

German troops crossed the border into Poland on September 1, 1939, which marked the official start of World War II. This invasion followed the signing of the Molotov-Ribbentrop Pact between Germany and the Soviet Union, allowing for the division of Eastern Europe. The swift military action employed by Germany, known as Blitzkrieg, led to the rapid fall of Poland and triggered declarations of war from Britain and France.