Name one war fought by the United States in the 1800s.

The Civil War is a significant conflict in U.S. history that was fought from 1861 to 1865, marking a pivotal moment in the nation’s development. This war was largely centered on the issues of slavery and states' rights and ultimately aimed to determine the fate of the Union and the preservation of the United States as a unified nation. The conflict involved numerous battles and significant political and social implications, leading to the abolition of slavery and Reconstruction efforts following the war.

The other wars mentioned, such as the Revolutionary War, were fought in the late 18th century, while World War I occurred much later, from 1914 to 1918, with the United States entering the conflict in 1917. The Spanish-American War, although part of U.S. military history, took place in 1898 and, while also significant, is not as central to the key struggles that defined the United States during the 1800s as the Civil War was. Therefore, identifying the Civil War helps underscore a major turning point in American social and political life in that century.