Which war was fought by the United States in the 1900s?

The Spanish-American War was fought by the United States in the late 19th century, specifically from April to August 1898. Although it technically falls at the boundary of the 1900s, it had significant implications that carried into the early 20th century, influencing U.S. foreign policy and its position as a global power.

In addition, the War on Terror, initiated in the early 2000s, has ongoing implications for U.S. military and foreign policy, and it is characteristic of 21st-century conflicts rather than primarily 1900s conflicts. The War of 1812 and the Civil War occurred in the 19th century and are thus not relevant to the 1900s timeframe specified in the question.

Thus, the Spanish-American War stands out as the war recognized within the context of this time period.