Which major conflict was fought by the U.S. in the 1800s?

The Civil War was a pivotal conflict in U.S. history that occurred from 1861 to 1865. This war was primarily fought over the issues of 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 not only preserved the Union but also resulted in the abolition of slavery with the passage of the 13th Amendment.

In contrast, World War I took place from 1914 to 1918, making it a key international conflict but outside the 1800s timeframe. The Spanish-American War occurred in 1898, which is at the very end of the 19th century, while the Mexican-American War took place between 1846 and 1848, which, although significant, is often overshadowed by the larger implications and outcomes of the Civil War in the context of American history. Thus, the Civil War is distinctly marked as the major conflict fought by the U.S. within the 1800s.