Michael N. answered 06/14/21
Georgia Tech Grad and Medical Student for Math, Science, and AP Tutori
When looking at America's role in WWII, we see that they were a pivotal source of equipment and fresh manpower. The nations of Europe were tired and whittled down by the end of WWII. With the US physically untouched by the destruction of the war, they were at the forefront of power with much of the allies relying on the United States for fuel their war machine and/or feed their populations.
Following the war, the US was unscathed by the destruction that impacted much of Europe, Asia, Africa, and the Middle East. They were able to step in via the Marshall Plan and provide aid in the form of military protection, food, and money to rebuild, allowing the Americans to gain a sphere of influence in each countries politics and decisions. In addition, America had one of the largest standing armies at the end of the war, so it was either fall in and get rewarded or fall out. Moreover, faced with the spread of communism and the iron fist of the Soviet Union under Stalin, countries opted into the protection of the United States' sphere of influence out of real and projected fear (created by the Americans) of communism spreading.
By rebuilding Europe and offering protection, the United States was able to form its own unofficial Empire through the power and favors they held.