Final answer:
The breaking of the non-aggression pact between Nazi Germany and the USSR, known as the Nazi-Soviet Pact, changed the course of World War II, leading to the defeat of Germany by opening the Eastern Front and marking the Soviet Union as an essential ally against the Axis powers.
Step-by-step explanation:
The breaking of the non-aggression pact, known as the Nazi-Soviet Pact, was significant because it changed the course of the war in Eastern Europe. When Adolf Hitler violated the pact by launching Operation Barbarossa in June 1941, it signalled a shift in the war's dynamic. Instead of being able to focus solely on Western Europe, Hitler now faced a powerful enemy in the Soviet Union, leading to a long and brutal Eastern Front. This violation ultimately led to the defeat of Germany, as the Soviets fought tenaciously, eventually pushing back the German forces all the way to Berlin.
The non-aggression pact initially allowed Hitler to avoid the mistake of fighting a two-front war, which was crucial for Germany's early advances. However, with the invasion of the Soviet Union, this strategic advantage was lost, forcing Germany to stretch its military resources. The Battle of Stalingrad was a major turning point, where the Soviet victory marked the beginning of the pushback against German forces. Moreover, the Soviet Union proved to be a formidable adversary, disproving Nazi racial ideologies that underestimated their capability for resistance.