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

The Civil War is the correct answer because it was a significant conflict in United States history that took place primarily from 1861 to 1865. This war was fought between the northern states, known as the Union, and the southern states that seceded from the Union to form the Confederacy. The central issue of the Civil War was primarily over the contentious topic of slavery and its expansion into the newly acquired territories. This conflict not only shaped the future of the nation but also led to major changes in federal and state relations, including the eventual abolition of slavery through the 13th Amendment.

In contrast, the Revolutionary War occurred in the 18th century, specifically from 1775 to 1783, which is outside the specified time frame of the 1800s. World War I and World War II took place in the 20th century, with World War I occurring between 1914 and 1918 and World War II from 1939 to 1945, thus also falling outside the 1800s.