I know this is going to kill my mod points, but I'll throw it out anyway...
What if WWII wasn't the worst thing in the web of possible 20th century events? What if there was no WWII and the Cold War started pre or early-nuke with a (mostly) demilitarized Europe and America had minimal troops and arms factories setup (as is the case in almost every "somebody went back in time and killed Hitler" story)? What if the European powers continued pointless and frequent war, treating it as a sport as they did in the 19th century and before?
If you look at WWII as the almost-inevitable epilogue to WWI due to the terms put on Germany, WWI/WWII were sort of the "War to end all wars [in Europe]" (ignoring smaller, regional conflicts). No one has invaded London, Paris, Berlin (although it was divided), Brussels, or Rome in almost 70 years.