I've come up with a "butterfly effect" alternate history for World War Two. This is that Edward VIII never meets Wallis Simpson and thus never has his affair and marriage with her. As a result, he doesn't abdicate and remains King when World War Two starts.
Being a constitutional monarch, this changes none of the broad strokes of diplomacy or warfare up until the fall of France and the evacuations at Dunkirk. At this point in time Chamberlain resigned as Prime Minister and there was great debate as to who should become the new Prime Minister. The two choices were Winston Churchill (if the British Empire wanted to keep on fighting) or Lord Halifax (if they instead wanted to negotiate a peace with Nazi Germany). The debate raged behind the scenes for ten days and in the end the King was credited for having tipped the decision in favor of Churchill. So in this scenario Edward (who disliked Churchill and who was mildly pro-Nazi) tips the decision the other way and Lord Halifax becomes Prime Minister.
Hitler actually wanted such a peace from the British (saw no point in fighting the British Empire and felt they had a fair dash of Aryan blood). So Britain makes a peace (some war reparations and a few colonies that were German prior to the end of World War One) and Nazi Germany is left to consolidate Europe and prepare to attack the Soviet Union.
Meanwhile, Lendlease never really gets going but the USA is still confronted by Imperal Japan. When Pearl Harbor or its equivalent comes, America can only declare war on Japan because the Germans don't declare war (their beef with the USA was all the assistance we gave the British in 1940 and 1941). At some point the USSR and Nazi Europe go to war and who knows who wins (but it's very close). The United States beats Japan and is thus left facing a totalitarian Europe of the Left or Right, with a nervous British Empire stuck between two superpowers.