The main advantages of the Allies over the Central Powers was that they had numbers on their side, and were surrounding them.
Germany and Italy were surrounded on pretty much all sides by the forces of the Allies, which at the end of the war turned to be crucial after they suffered defeats in some battles, and then were cornered. The Japanese too were surrounded on all sides by the forces of the Allies, not to mention that they were heavily outnumbered after the Allies organized properly, which again led to gradual winning of position, battle by battle, until the atomic bombs were used for speeding up the procesa and avoiding more casualties.