Why Did The United States Adopt Internationalism After World War I?

What was, in your opinion the most important factor that led the United States to abandon its policy of isolationism and adopt internationalism after Second World War?
When the second world war ended, US got stuck in such a network of alliances and signing of international organizations that her retirement to her continent and adopting the policy of isolationism become quite impossible for her. But the most outstanding reason, that drew out her out from her isolation was, the rise of her potential rival, USSR. USSR had adopted the policy of communism through which it wanted to upset the world order and was adamant to communize the whole world. Therefore to contain it, there was no capable power in sight, as the war had ravaged the rest of powerful states to such an extent that they were not even able to feed their people. In that case of scenario, US abandoned her policy of isolationism and entered the world politics to neutralize the red menace.
After the Second World War, when the treaty of Versailles was signed and Europe had got the sigh of relief, US seemed no reason to entangle herself anymore in the European
They were to avenge the Pearle Harbor; to thwart Nazi idea to engulf the whole of Europe; to put a reign to imperialist Japan; to make the world safe for democracy and to establish a world organization which could replace the League of Nations. But these objectives were more or less temporary and after completing them US could have opted to retire back to the policy of isolationism, but the real factor which held her engaged in the politics of the world for so long was actually its rivalry with the USSR. If she had not contained the communist propaganda, the USSR would have no difficulty in propagating its policy of communism and majority of the world would have been in the grip of communism and USA would not have been in such a powerful position, which she is in,