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

The Civil War was indeed fought by the United States in the 1800s, specifically from 1861 to 1865. This conflict primarily arose from tensions surrounding slavery and states' rights, culminating in the Southern states seceding from the Union. The war had profound social, economic, and political implications, leading to the preservation of the Union and the abolition of slavery.

In this context, while the Revolutionary War and the War of 1812 are significant conflicts in American history, both occurred prior to the Civil War. The Revolutionary War took place from 1775 to 1783, and the War of 1812 spanned from 1812 to 1815. World War I, on the other hand, was fought in the 20th century, specifically from 1914 to 1918, making it unrelated to the scope of wars fought in the 1800s. Understanding the timeline of these wars helps to clarify why the Civil War is the correct answer for conflicts specifically occurring in that century.