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

The Civil War is recognized as one of the most significant conflicts in American history, fought from 1861 to 1865. This war was primarily centered around the issues of slavery and states' rights, which led to a division between the Northern states (the Union) and the Southern states (the Confederacy). The Civil War had profound effects on the nation, resulting in the preservation of the Union and the abolition of slavery through the Emancipation Proclamation and the subsequent 13th Amendment.

The other wars mentioned occurred outside of the 1800s. World War I took place from 1914 to 1918, World War II spanned from 1939 to 1945, and the Vietnam War occurred from the late 1950s to the mid-1970s. Therefore, only The Civil War fits the criteria of a conflict fought by the United States during the 1800s.