Reading my book about the development of nuclear diplomacy in 1945, the author mentions that Roosevelt put forward in The Four Freedoms and The Atlantic Charter that people have, as a fundamental right, the right to determine their own self-government.
Except in 1865, when that would have led to the partial dissolution of the United States. And in 1812 when the New England States wanted to secede. And today, when Washington and Oregon and California and probably many other states have secessionist movements.
If you follow that logic down the rabbit hole, you'd have very tiny, very insular and ideological nation states at constant war with each other. Large nations made up of diverse people and held together by tenuous threads of national identity, are probably the most stable / least warlike.
Another common theme in this book is the idea of "world government" where the nations all have merged to control the unimaginable horror of a nuclear war. Putting aside the knee-jerk nationalism that prevents any nation from agreeing to submitting to a higher authority, and the corrosive effect of "men of ambition" who would try to use such an arrangement for their own benefit, world government makes a lot of sense. Or as an interim, bestowing enough power on The United Nations that rogue states could be controlled.
Idealism. You may say I'm a dreamer, but I'm not the only one.