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

The Civil War was fought by the United States in the 1800s, specifically from 1861 to 1865. This conflict arose primarily from tensions surrounding slavery and states' rights, leading to a division between the Northern states (the Union) and the Southern states (the Confederacy). The outcome of the Civil War was pivotal in shaping the nation's history, resulting in the preservation of the Union and the abolition of slavery through the 13th Amendment.

World War II and the Vietnam War occurred in the 20th century, with World War II spanning from 1939 to 1945 and the Vietnam War occurring primarily during the 1960s and 1970s. The Spanish-American War took place in 1898, but it is only relevant as a conflict towards the end of the 19th century, rather than within the broader context of the Civil War's significance in the 1800s. Therefore, the Civil War stands out as the most significant war that the United States fought during that century.