Not really true.
The UN started out in 1945 as the anti-fascist alliance defined at Yalta. The USSR was one of the legs of that alliance. But then the Cold War brewed up when Stalin installed puppet dictatorships throughout eastern Europe. NATO was a direct response to fear of invasion by a bloodthirsty dictator who'd accidentally ended up on the winning side (remember he'd started out allied with Hitler? Cf. Molotov-Ribbentrop Pact and Invasion of Poland).
@cstross @pettter let's not forget what US has been up to just to give a bit of a balance to your narrative https://www.counterpunch.org/2020/10/16/the-u-s-did-not-defeat-fascism-in-wwii-it-discretely-internationalized-it/