Which of the following wars was fought by the United States in the 1900s?

World War I, which occurred from 1914 to 1918, is a significant conflict in which the United States actively participated, officially joining the Allies in 1917. This war shaped not only U.S. military strategy but also had profound implications for global politics and the eventual establishment of international organizations aimed at promoting peace, such as the League of Nations.

The other wars listed, such as the American Civil War and the Mexican-American War, took place in the 19th century, while the Korean War lasted from 1950 to 1953. Thus, World War I is the only conflict from the options that falls squarely within the 1900s, making it the correct choice for this question.