Why were the Allied Powers unable to stop the German,

Japanese, and Italian Aggression?

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Explanation:

There was no such thing as Italian Aggression. I'm assuming your teacher is asking about the Italian invasion on Ethiopia/Abyssinia. It was disastrous for the Italians, they took almost a whole year to destroy an army that was mainly armed with bows and arrows and spears. The Allied Powers couldn't stop this invasion because they didn't care about it. The only country to support Ethiopia was Germany, through shipments of half-tracks and tanks.

The Germans steamrolled through western Europe because of the newly developed blitzkrieg. The Germans would send a bunch of planes into combat first and bomb the defending cities'  factories, supplies, and not allowing the defending armor and infantry to set up in defensive positions. Afterwards, the Germans would steamroll their way through with their combined attack of infantry and armor. The Germans would develop encircling tactics as well -- used in Dunkirk and the Russian campaign -- forcing troops to surrender or be annihilated, weakening the enemy force.

The Japanese bombed Pearl Harbor, destroying 18 ships of multiple kinds. By bombing most of the pacific fleet, the Japanese eliminated the US presence in the Pacific for a while, allowing them to take over most of the western controlled islands in the Pacific. All the Japanese had to do was overwhelm the troops on the islands, which they did with banzai charges. This only worked early in the war due to China's lack of good military leaders and smaller forces occupying the islands.