In 1939, Germany and the Soviet Union shocked the world with a non-aggression pact. Adolf Hitler had long viewed the decision to fight on two fronts in World War I as Germany’s biggest mistake.

Hitler had long seen the Soviet Union as an enemy, calling for an invasion of the Soviet Union in his book, Mein Kampf. His fear of fighting a two-front war, though, prompted him to try to secure a non-aggression pact, especially after the reaction to the annexation of Czechoslovakia in the spring on 1939.

The plan seemed to work. His spring 1940 invasion into Western Europe succeeded, bringing the fall of France and the Low Countries. German U-boat wolf packs were successfully attacking convoys to the United Kingdom. The Britain and its Allies were in desperate straits, even with America providing Lend-Lease support of weapons and equipment to fight Axis forces.

As the summer of 1941 approached, Hitler was ready to turn on Russia. The initial plan to invade in May was delayed by the need to help Mussolini take Greece and Crete. As a result, on June 22, 1941, the Germans launched Operation Barbarossa – the invasion of the Soviet Union with over three million troops. What was thought to be an easy victory instead fell short less than 20 miles from Moscow, and turned into a war of attrition.

Ultimately, England also held out, and Hitler soon found himself facing what he feared the most: A two-front war.