I think it was just that post-war they were seen as a vital player on the continent, nobody wanted yet another war with Germany and everyone knew -esp Britain and the USSR that France would be the "best" buffer. They could also keep an eye on the pesky fascist/marxist movements for the rest of the council, especially in Italy and Spain. Although if Roosevelt had lived a little longer who knows? Maybe us and the french would have been cast off.

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The U.N. wasn't created to be "fair" and it certainly wasn't supposed to be a Democracy. This is why H.G. Wells was among those who helped draft the founding documents of the organization.