I have often wondered what would have happened, given one mere change to WWII: that is, that Hitler had not invaded Russia, and instead had (more wisely) decided to take out Britain first.
He would have won, and he probably would have been able to keep most of europe and north africa and the middle east.
The mind boggles at how the 20th century would have been different.
That was Hitler's original plan as laid out in "Mein Kampf" and his other prewar comments. I'm not sure why he didn't stick to it. I think part of it was that he underestimated how long it would take to knock out Britain--he was relying on estimates by Charles Lindbergh, who mistakenly predicted that the Luftwaffe could wipe out London without any serious opposition. Also Hitler was hoping the US would stay out of it, and was reinforced in this hope by encouragement from Joseph Kennedy (JFK's father), who exaggerated the amount of anti-Semitism in the US and assured Hitler that the US wouldn't care if he got rid of the Jews. Hitler's generals didn't want to fight the US and most of them were stunned when Hitler declared war on America after Pearl Harbor. Hitler was prone to make wild split decisions. When his generals warned him against invading Russia in winter, he answered something to the effect of, "You just fight--I'll take care of the weather!"