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

The Civil War is a significant conflict that took place from 1861 to 1865, and it is one of the most notable wars fought by the United States in the 1800s. This war was primarily fought over issues such as states' rights and slavery, which had created deep divisions between the Northern states (the Union) and the Southern states (the Confederacy). The Civil War had far-reaching consequences for the nation, including the preservation of the Union, the abolition of slavery, and significant social and political changes in American society.

The other conflicts mentioned in the choices occurred outside the 1800s. World War I took place from 1914 to 1918, the War on Terror began in the early 2000s, and the Spanish-American War occurred in 1898, but it is not as directly defined within the broader context of 19th-century America as the Civil War. The prominence and impact of the Civil War make it the clearest and most relevant choice for a war fought by the United States during the 1800s.