Once world war 1 was over, they had the versailles treaty, which weakened the German military, but ended world war 1. Along with this, a weak government was made for Germany which showed potential for anyone who may of had any idea of what to do with the country, this being one of the reasons Hitler was elected to become the leader of the Third Reich. Another thing that made world war II inevitable was the great depression, the great depression severely lowered the economy world wide. Along with that, with communism had taken Russia over, which had allowed the U.S.S.R to form, with this being formed another power hungry leader had been chosen, Joseph Stalin. Along with that, Japan was also looking for any sort of power, so they invaded the Korean Countries which raised tensions with China, and the U.S.S.R. (Sorry this may not be much, but this is what I could come up with.)
