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

The Civil War is an appropriate choice as it was a significant conflict fought in the 19th century, specifically from 1861 to 1865. This war arose from deep-seated issues including states' rights, economic disparities, and the moral quandary of slavery, ultimately leading to a confrontation between the Northern states (the Union) and the Southern states (the Confederacy). The Civil War had lasting effects on the United States, leading to the abolition of slavery, the strengthening of federal authority, and significant socioeconomic changes.

Other wars mentioned, such as the Revolutionary War and the Spanish-American War, occurred outside of the 1800s. The War of 1812, while also a conflict in the 19th century, is distinctly different in scope and impact when compared to the Civil War, which reshaped the nation in profound ways. Hence, the emphasis on the Civil War as a pivotal moment in U.S. history makes it the correct answer for this question regarding wars fought in the 1800s.