Which war did the United States participate in during the 1900s?

World War I is a significant conflict in which the United States participated during the 1900s, specifically from 1917 to 1918. This global war involved many nations and marked a pivotal moment in world history. The U.S. entered the war primarily due to submarine attacks on American merchant ships and a desire to support the Allies against the Central Powers, which included Germany, Austria-Hungary, and the Ottoman Empire.

The participation of the United States in World War I had lasting effects on both the country and the world, leading to the establishment of the League of Nations and setting the stage for future international relations. The war also had a significant impact on American society, including economic changes and shifts in social dynamics as the nation mobilized for war.

The other options represent wars that occurred outside the specified timeframe of World War I. The Civil War occurred in the 19th century, while the Korean War happened in the early 1950s, and the Afghanistan War began in 2001, well into the 21st century. Thus, World War I is the correct answer as it is the notable military engagement of the 1900s that involved the United States.