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

World War I is indeed a significant conflict that the United States participated in during the 1900s. The war lasted from 1914 to 1918, although the United States entered the conflict later, in 1917. It was a global war that involved many of the world's great powers and had a profound impact on the course of history. The war is particularly noted for leading to significant changes in many countries, including the United States, which emerged from the war as a more prominent global power.

The other options, while important conflicts in U.S. history, do not fit the specified criteria of being fought in the 1900s in the same context. The Civil War occurred from 1861 to 1865, the Spanish-American War was fought in 1898, and the Gulf War started in 1990, which may lead to confusion. Each of these wars plays a crucial role in understanding U.S. military history, but World War I is the correct answer given the time frame specified.