jun 22, 1941 - German Invasion of USSR
Otherwise known as "Operation Barbarossa", Hitler believed that he could take over Russia using Blitzkrieg with the same level of success as with France and the Low Countries. This was in direct violation of the Non-Aggression Act and led the USSR to switch sides and fight for the Allies. Germany lost terribly in this battle.
Added to timeline:
For Mr. Kelly's American History II Honors Class