During the 2nd World War, President Roosevelt's Atlantic Charter had committed the USA to dismantling the British and French empires in Asia, although not to dismantling their own.
Roosevelt had struck up an apparently close relationship with Stalin, and regularly humiliated Churchill. Quite rightly, he decided that the USA and Russia were now the most powerful countries in the world and European countries had worn themselves out. He disliked European imperialism but failed to see the USA's own example or that Russia was building one up in Eastern Europe.