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

The Civil War is indeed a significant conflict that was fought in the United States during the 1800s, specifically from 1861 to 1865. This war was primarily centered around the issues of slavery and states' rights, and it involved the Northern states (the Union) battling the Southern states (the Confederacy) that had seceded from the Union. The outcome of the Civil War had profound effects on the nation, including the preservation of the Union and the abolition of slavery.

While the other wars listed are important in U.S. history, they do not fit the timeframe of the 1800s as precisely as the Civil War does. The Revolutionary War took place from 1775 to 1783, and World War I occurred much later, from 1914 to 1918. The War of 1812 did occur in the 1800s, but the significance of the Civil War in terms of its impact on the United States makes it a prominent answer for a war fought in that century.