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

The Civil War is indeed a significant conflict fought by the United States during the 1800s, specifically from 1861 to 1865. This war was primarily centered around the issues of slavery and states' rights, resulting in a profound impact on the nation’s history and its future direction. The conflict pitted the Northern states (the Union) against the Southern states (the Confederacy) and ultimately led to the abolition of slavery and the preservation of the Union.

The other wars listed occurred at different times. The Revolutionary War took place before the 1800s, from 1775 to 1783, and it involved the thirteen American colonies fighting for independence from British rule. The War of 1812 occurred in the early 19th century but still falls before the Civil War timeline, lasting from 1812 to 1815 primarily between the United States and Great Britain. World War I was fought much later, from 1914 to 1918, and involved many nations globally, not just the United States. Therefore, within the context of wars fought by the United States during the 1800s, the Civil War is the correct answer.