Since the end of the nineteenth century, it had been clear that the resource-rich and rapidly industrializing United States was the world's rising superpower, but the US was reluctant to adopt this mantle. This was rooted in history: the country took to heart George Washington's advice in his Farewell Address that it steer clear of involvement in European politics and conflicts.
The US did enter belatedly into World War I, but after that war, it retreated back into isolationism, stunningly refusing to enter the League of Nations, despite this organization being close to the heart of President Woodrow Wilson. The US was equally reluctant to join in World War II (though FDR knew this was inevitable) until the country was directly attacked at Pearl Harbor.
After the War ended in 1945, however, the US finally fully accepted its role as a world leader. This was an enormous change in its orientation to international politics. The US was at the forefront in establishing the UN, headquartering it in New York City, in creating NATO, and in crafting the post-war world order. It also acted aggressively as a global policeman trying to halt the spread of communism.
Domestically, the War jolted the country out of the Great Depression. During the war, a labor shortage gave unions a strong bargaining hand, as did sympathetic policies on the part of the Roosevelt administration. After the war, despite fears of a return to depression, the US became the world's greatest industrial engine, and business boomed.
High wages and good benefits, along with a robust economy, led to unprecedented levels of wealth flowing into the hands of ordinary Americans, leading to thirty or more years of prosperity and growth.