Which of the following wars was fought in the 1800s?

The Civil War is the correct choice because it was fought from 1861 to 1865, making it a key conflict in American history during the 1800s. This war was primarily about the issues of slavery and states' rights, leading to a confrontation between the Northern states (the Union) and the Southern states (the Confederacy). The significance of the Civil War lies in its profound impact on the nation's social, political, and economic landscape, ultimately leading to the abolition of slavery and significant changes in the relationship between the federal government and the states.

In contrast, the Vietnam War took place primarily in the 1960s and 1970s, the Second World War occurred from 1939 to 1945, and the Korean War was fought in the early 1950s, all of which are outside the 1800s timeframe.