What Germany did that started World War II was invading Poland in 1939.
Explanation:
The Invasion of Poland by Nazi Germany, the Soviet Union and a minor Slovak force as triggered World War II. Germany attacked Poland on September 1, 1939. On September 3, France and Britain declared war on Germany. Before that, Germany and the Soviet Union had signed the Molotov-Ribbentrop pact, which among other things divided Poland between those countries. On September 17, Poland was invaded by the Soviet Red Army.
