Why did the Soviet Union join the Allies?

A. Stalin was afraid of Hitler.

B. Hitler had broken a pact and attacked.

C. Hitler was becoming too powerful in Eastern Europe.

D. Stalin wished to take control of territories captured by Germany.

B.

Explanation:

In June 1941, Hitler broke the non-aggression agreement with Stalin and Germany invaded the Soviet Union.

B. Hitler had broken a pact and attacked.

Explanation:

On the 23th of August, 1939 (some days before the beginning of WWII) Hitler and Stalin signed a German-Soviet Nonaggression Pact, according to which the two nations would not execute any military attack on each other for 10 years. This was a strategic move on both sides, since Stalin saw it as an opportunity to stay out of conflict and reinforce the Soviet military, and Hitler wanted to invade Poland without opposition. But in 1941, after invading Poland, Germany broke the pact, starting its invasion over Soviet territory. This forced Russia to join the Allies in an effort to stop a very powerful Germany.