How did Hitler’s breaking of the Hitler Stalin pact of 1939 change the war in Europe

It caused significant losses on the side of the Germans, as well as the Soviets.

Explanation:

Although both sides suffered heavy losses, Germany was not able to replenish the resources and manpower lost during the invasion of the Soviet Union. However, the exact opposite is true for the Soviets. After Stalin decided to fight back against Germany's invasion, the Russian industrial complex sky-rocketed, massively outgunning Germany's struggling forces. Ultimately, without the Soviet's intervention, it's more than likely the war may have lasted many years longer than it did. The invasion of  Russia was a huge blunder for the Germans as it opened up a second front against an enemy Germany was not wholly ready to combat.

a. poison gas was sprayed over battlefields by airplanes to weaken enemy positions before major attacks.