Which event is classified as a war fought by the United States in the 1800s?

The event classified as a war fought by the United States in the 1800s is the Civil War. This conflict spanned from 1861 to 1865 and was primarily fought over issues such as states' rights and slavery. It was a pivotal moment in American history, resulting in significant societal changes and the eventual abolition of slavery. The Civil War involved numerous battles and was primarily fought between the Northern states (the Union) and the Southern states (the Confederacy), marking a key period of internal conflict in the United States.

While the War of 1812 also occurred in the 1800s, it started earlier, in 1812, and was ended before the Civil War began. World War I took place in the early 20th century, from 1914 to 1918, and the Spanish-American War occurred in 1898, which technically belongs to the late 1800s but is not as significant in the context of the 1800s as the Civil War. Therefore, the Civil War is the overarching conflict that defines the U.S. experience in the 1800s.