What is one war fought by the United States in the 1800s?

The Civil War is indeed a key conflict fought by the United States in the 1800s, specifically from 1861 to 1865. This war was primarily a result of deep-seated issues surrounding slavery, states' rights, and regional differences between the North and South. The Civil War was crucial in defining the nation, addressing the complexities of federalism, and ultimately leading to the abolition of slavery with the passage of the 13th Amendment.

In contrast, the Revolutionary War took place in the late 18th century, ending in 1783. The World War refers to the global conflicts that occurred in the 20th century, specifically World War I and World War II. The Korean War, a significant conflict, occurred in the early 1950s and is outside the 1800s timeframe. Each of these wars played critical roles in American history, but they did not occur during the 19th century, making the Civil War the correct choice for this question.