WW2 was clearly a factor in putting the USA in the position of superpower.
In 1939, the USA had a weaker, in number, army than Romania. They had a powerful fleet , however, and aviation was OK: reliable airplanes, with useful performances.
In 1945, they are first in aviation, navy, and second in land army after the Soviet Union.
Economic and industrial:
In the meantime, a powerful industry has been put to its maximal production rate, putting many women to work. Thus, the US enhanced their workforce.
Economical power is vastly supported by the financial edge, with gold reserves. They managed well this power when they achieve to put into service the Bretton Wood system.
Multiple and reliable allies in Europe, even if they had some trouble with France. Communist parties failed to be an efficient counterpower in most european countries.
Strong alliances in South America. Good partners in the Middle East, from Saudi Arabia to Israel a few decades later.
From Captain America to movies.
Food power: Important thing. The Marshall plan was for a great part about feeding the Europeans, and America achieves to do that. They sold a lot of food, seeds and fertilizer.
However all this power was a little hampered by the vigorous fights in Asia: China, Korea and later Vietnam. USA lost there a lot of bases, and a lot of cultural power fled to non governemental forces (like the hippie movement), thus levering down the American edge in the cultural influence.