What was a key reason that the allies were able to stop the german and the japanese forces?

One reason is the U.S. entered the war with fresh troops and arms. Also language is important because the British and Americans spoke the same language on the Western front while Russia was pushing from the East with huge amounts of manpower the Germans and Italians did not speak the same language and Italy surrendered early in the war. The Japanese were fighting the U.S. in the Pacific and were not able to help. 

Iagree ☝️ with that dude