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

The Civil War is indeed one of the major conflicts fought by the United States during the 1800s. It took place from 1861 to 1865 and was primarily a result of tensions surrounding slavery and states' rights. This war marked a pivotal moment in American history, leading to significant social, economic, and political changes, including the abolition of slavery.

The other wars mentioned in the options each occurred at different times - the War of 1812 occurred from 1812 to 1815, the Spanish-American War took place in 1898, and World War I was fought between 1914 and 1918. Therefore, while the Civil War gained prominence in the mid-to-late 1800s, the other conflicts represent different historical contexts beyond that century.