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

The Civil War is indeed a significant conflict that was fought by the United States during the 1800s, specifically from 1861 to 1865. This war was primarily centered around issues of slavery, state rights, and the preservation of the Union. It was a pivotal moment in U.S. history as it not only addressed the moral and political issues surrounding slavery but also led to major changes in the structure of American society and governance.

The other conflicts mentioned occurred at different times. The Revolutionary War, which won independence from British rule, took place in the late 18th century, while the War of 1812 occurred in the early 19th century. World War I happened much later, from 1914 to 1918. Each of these wars has its own historical significance, but the Civil War is the only one listed that directly fits the time frame of the 1800s and had a profound impact on the trajectory of the United States.