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

The Civil War is a significant conflict that occurred in the United States during the 1800s, specifically from 1861 to 1865. It was fought primarily over issues including states' rights and slavery, leading to a division between the Northern states (the Union) and the Southern states (the Confederacy). This war had profound impacts on the nation, addressing fundamental issues regarding citizenship, freedom, and the role of government, ultimately shaping the future of the United States.

The other options refer to wars that took place outside the 1800s. World War I occurred from 1914 to 1918, while World War II spanned from 1939 to 1945. The Spanish-American War was fought in 1898 but is typically regarded as a war that marks the end of the 19th century rather than the majority of the 1800s, making the Civil War the most representative example of a war fought during that century. Thus, the Civil War is the best answer for a war fought by the United States specifically in the 1800s.