Pity that we let it get away from us. It's hardly the most popular thing in some parts of the world these days - that is, when people think about it at all.
The United Nations and United States flags fly equally, but from their own flagpoles, in San Francisco's United Nations Plaza.
Plenty of people dislike or distrust the UN. I won't pretend there aren't perfectly valid reasons to do so, but it almost seems as if the loudest opponents aren't basing their opposition on things like its problems with peacekeeping or corruption in Turtle Bay, but on black helicopters and the New World Order. This is, realistically, bunk - the UN has less teeth than a pitcher plant. The only "military" it has is those peacekeeping forces that have been seconded to it. We could've gone another way, of course - speculatively speaking, after the Second World War, the United States could've turned its nuclear arsenal over to the new organization as a guarantee that no country would be allowed to start the Third World War.
Whether it would have been considered politically feasible was another matter entirely; aside from Robert A. Heinlein suggesting it in one of the essays he included in Expanded Universe, I've never encountered it having ever been advanced as a serious policy. But then, there were only four years in which it was even an option; once the Soviet Union exploded Joe-1 in 1949 the jig was up, as Stalin would never have handed his bombs over to a tool of the Western capitalist oppressors.
Now, though - if it was unfeasible in 1945, it would be purely impossible in 2011. Twenty years after the Cold War, everyone would rather we all forget about those thousands of warheads that are still kicking around and good riddance to them. It's true, the threat of nuclear annihilation did end up guaranteeing the general peace, but it was pretty damn dicey for a while there. A world where the United Nations maintained a nuclear monopoly would be a world that didn't have to worry about nuclear war. Granted, though, such a situation easily leads in to a world where the United Nations becomes a world government - because he who has the nukes, makes the rules. could be an interesting story setting. Regardless--
You can also read the Preamble to the United Nations Charter at United Nations Plaza, and there's one part of it that really struck me as I walked along: its intention "to ensure, by the acceptance of principles and the institution of methods, that armed force shall not be used, save in the common interest" - that is, the United Nations comes along and tells its members that they can't go around declaring war for the lulz anymore.
Fighting is an intrinsic part of human nature. Hell, it may well go back to the time before there were humans; groups of chimpanzees have been observed to go to war with each other over territory. Sure, every once in a while there were attempts by some group or another to try and keep Europe from being ground into dust for a generation or two, but they generally did not produce lasting results. The United Nations, on the other hand, came from the crucible of the most destructive conflict in all of human history; it meant to stick.
Stick around, at least - one might argue that the UN ideally functions as the world's conscience, something which the right people can ignore. The United States certainly wasn't listening to the United Nations when it invaded Iraq back in 2003, just as the most recent example I can think of. The UN's presence in the Korean War was possible only because the Soviet Union was boycotting proceedings at the time and thus not around to veto Resolution 84.
I'd like to think this is baseless speculation, that the real reasons have more to do with things like perceptions of a more powerful UN overriding national sovereignty, and whether its dedication to the interests of the people of the world is questionable; because otherwise, I really worry it might come down to, somewhere in the reptile brain, "Who the fuck do they think they are to tell us we can't kill the bastards?"