After WWII, the United States was on top of the world. It was the only major war power to not have suffered fighting on its land, plus it had lost a relatively smaller number of soldiers (418,000) compared to some of its allies (USSR: probably around 10,000,000 soldier deaths and an equal number of civilians).
It thus dominated war-ravaged Asia and Europe politically and economically and used this power to shape much of the post-war world and agreements.
The United States joined the United Nations, which meant that that UN, unlike the post-WWI League of Nations, was a bit more successful in its peacekeeping goals. The US also controlled the post-1945 economic system set up at the Bretton Woods Agreement: the US was a major voice in the World Bank and International Monetary Fund set up by that conference. Plus, the dollar was the currency of choice and the medium of exchange for much of the world’s economy.
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The US was the only country with nuclear weapons (so far…)—this is called a nuclear monopoly—and used those weapons to face off against a former ally with a large army that the US began to distrust more and more, the USSR.
This next chapter of tension with the Soviet Union would be known as the Cold War (see Period 8 guide).
The war-ravaged condition of Asia and Europe, and the dominant U.S. role in the Allied victory and postwar peace settlements, allowed the United States to emerge from the war as the most powerful nation on Earth.