I would make sure Germany never forms, because it started BOTH world wars.
If I've learned anything from Red Alert, it's that Germany is the lesser of two war powers, without Germany, Stalin and his Soviet Union would conquer all of Europe unopposed. You would effectively be creating an even more horrible super dictator and cause WW2 to last beyond the 1960s.
Not to mention that Germany was merely reactionary when it "Started" both world wars. WW1 was just a game of alliances, but because France and Britain hated Germany for some reason, they forced it to suffer some pretty extreme losses as though it had instigated the war for funsies. So obviously when a dictator named Adolf rose to power, promising to get revenge and make Germany the super power it once was, you can imagine that this wasn't met with much resistance. France and Britain picked on Germany, and Germany 20 years later blew their countries to oblivion with bombs and blitzkriegs.