So if America was isolationist before the war- and I've argued that it actually wasn't really- after the war it certainly wasn't. FDR took a very active role in planning for a more peaceful and prosperous post-war world. And conferences at Tehran, Yalta, and Potsdam clarified war aims, and established the idea that Germany would be divided and Nazis tried for war crimes.
These conferences also laid the foundation for the Cold War in allowing Soviet influence in Eastern Europe, especially Poland, so that wasn't such a good thing.
But, the 1944 conference Bretton Woods in beautiful, freedom loving New Hampshire, established America's economic dominance as the dollar, which again would be backed by gold, replaced the pound as the main currency in international transactions. It also created the World Bank to help rebuild Europe and also to help developing countries and the IMF to stabilize currencies.
How well that's worked is debatable, but this isn't- the US became the financial leader of a global capitalist order. the US also took the leading role in establishing the United Nations at the Dumbarton Oaks in 1944. Why do we not have a UN commission on improving the names of historical events?
The goal of the UN was to ensure peace, and the US's position as one of the 5 permanent members of the Security Council signaled that it intended to take an active and leading role in international affairs. And we had to, because by the end of the war only the US and the USSR were powerful enough to have any influence.
So WWII ended the Depression and transformed American economy. It cemented the new definition of liberalism defined by the New Deal and opened up opportunities for diverse groups of Americans. It also transformed definitions of freedom both at home and abroad. I mean, even before the US entered the war it issued the Atlantic Charter along with Britain affirming the freedom of all people to choose their own government and declaring that the defeat of Nazi Germany would help to bring about a world of "improved labor standards, economic advancement, and social security".
At home and abroad WWII became a war that was about freedom, but was also about what Gunnar Myrdal called the American Creed- a belief in equality, justice, equal opportunity, and freedom.
I want to be clear that we have done a terrible job of living up to the American Creed, but the story of American history is in many ways the story of ideas pulling policy, not the other way around. American history is an economic and political and social history, but it is also a story about the power of ideas. and WWII helped clarify those ideas for America and for the world.