Are there any plausible ways to make the UN much stronger than it is today, where it acts more like a global government? Possibly with its own armed forces? What got me thinking about this was that I discovered that, in either 1946 or 47 President Truman proposed to give the US atomic arsenal to the UN.
So maybe a longer, more destructive world war 2 makes people more willing to give the UN real power, to prevent a new even more destructive war? Possibly with atomic weapons used by both sides, or maybe things break down as the nazis collapse and the west and the USSR duke it out, and after that war this more powerful alt-UN is formed?