Final answer:
The German invasion of the Soviet Union, Operation Barbarossa, broke the Nazi-Soviet Pact and united the Allied Powers.
Step-by-step explanation:
The invasion that broke a 'non-aggression pact' and brought together the three major Allied Powers was the German invasion of the Soviet Union, known as 'Operation Barbarossa'.
This invasion violated the German-Soviet Nonaggression Pact, also known as the Nazi-Soviet Pact, that Hitler and Stalin had signed in August 1939.
Operation Barbarossa began on June 22, 1941, and led to the Soviet Union formally joining the Allies against Germany.
The initial success of the German forces was quickly challenged as the Soviet army adapted and the harsh Russian winter set in, resulting in a significant turning point of the Second World War.