Name one war fought by the United States in the 1900s.

World War I is a significant war fought by the United States in the 1900s, specifically from 1917 to 1918 when the U.S. entered the conflict. This global war involved many nations and was marked by trench warfare, major battles, and various new military technologies. The U.S. joining this war played a crucial role in tipping the balance in favor of the Allies, contributing to the eventual defeat of the Central Powers.

In contrast, the other wars listed occurred in different centuries: the War of 1812 took place in the early 19th century, and the Civil War occurred in the 1860s. Although World War II was another major conflict involving the U.S., it began after World War I, starting in 1939. Therefore, World War I is an appropriate answer as a war fought specifically in the 1900s.