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

     Germany ran out of fuel and supplies for their ground and air forces. That is why they had to retreat from Russia, and stop bombing England. Then, the USA dropped "Fat boy" and Little boy" on Nagasaki and Hiroshima. That is how the Allies won. Also, big thanks to the Ruskies, it would have been really hard if they did not help the Allies.