Which war was fought by the United States during the 1800s?

The correct answer is the Civil War. The Civil War, fought from 1861 to 1865, was a pivotal conflict in American history, primarily centered around issues of slavery and states' rights. It resulted in significant changes to the nation's political landscape and led to the eventual abolition of slavery with the passage of the 13th Amendment.

While the Revolutionary War was indeed fought by the United States, it occurred in the late 18th century, specifically from 1775 to 1783. The War of 1812 was another important conflict that took place in the early 19th century, but the specific time frame of the Civil War makes it the most notable war in this context. The Spanish-American War, occurring in 1898, is also outside the scope of the question, as it took place at the end of the century and not during the 1800s in the same era as the Civil War.