Which of the following was a war fought by the United States in the 1800s?

The Civil War was a significant conflict fought in the United States during the 1800s, specifically from 1861 to 1865. This war was primarily focused on issues surrounding slavery and states' rights, leading to the secession of Southern states from the Union and the formation of the Confederacy. The war ultimately resulted in the preservation of the United States as a unified nation and led to the abolition of slavery through the 13th Amendment.

The other wars mentioned occurred in different time periods. The Spanish-American War took place in 1898 and is distinct from the Civil War, although both are part of the narrative of American conflict history. World War II occurred from 1939 to 1945 and was a global conflict involving many nations, contrasting sharply with the internal nature of the Civil War. The War on Terror began in the early 2000s and focuses on contemporary issues related to terrorism and military actions in various countries. Therefore, the Civil War is the only conflict listed that was fought in the 1800s, making it the correct answer.