Which war was fought by the United States in the 1800s?

The Civil War is indeed one of the significant conflicts fought by the United States in the 1800s, specifically from 1861 to 1865. This war was a pivotal moment in American history, primarily fought over issues such as states' rights and slavery. The conflict arose between the northern states, known as the Union, and the southern states that seceded from the Union to form the Confederate States of America.

The War of 1812 also holds importance, as it occurred between 1812 and 1815 and involved conflicts with Great Britain, yet it is classified as an 18th-century war. The Spanish-American War took place in 1898 and is outside the 1800s timeframe. World War I, vastly significant in global history, didn't occur until the early 20th century, starting in 1914, which further solidifies the exclusive belonging of the Civil War to the 1800s.

Thus, the Civil War is a correct representation of a war fought by the United States within the specified century, marking considerable historical significance in terms of social and political change in the nation.