How did the United States become important in world affairs?

America's rise to world-power status came in three stages:

1-As the newly emerged industrial giant in 1880-1910, American steel, grains, and crude oil output matched and exceeded that of Germany and England, enabling the United States to expand it's military. America's overwhelming and easy victory in the Spanish-American War forced the Europeans to consider the U.S. as equals.

2-The French and British armies had collapsed in WW1, and only American military might and industrial strength saved them from surrender in 1917. The world's post WW1 economy was dominated by the United States as the Europeans tried to rebuild their war-shattered economies and infrastructures.

3-WW2. Very simply, the United States industrial strength armed the world against the Germans and Japanese, and the subsequent Marshall Plan and it's equivalent in Japan cemented America's status as a superpower.