What major event happened during the 1800s?

The major event that occurred during the 1800s was the Civil War, which took place from 1861 to 1865. This conflict was primarily fought between the northern states (Union) and the southern states (Confederacy) that seceded from the Union. The war was driven by issues such as slavery, states' rights, and economic differences between the industrial North and the agrarian South. The Civil War had a profound impact on American society, resulting in significant changes, including the abolition of slavery and the strengthening of federal power over the states.

In contrast, the Great Depression occurred much later, in the 1930s; the American Revolution took place in the late 18th century, specifically from 1775 to 1783; and World War I occurred from 1914 to 1918, well into the 20th century. Therefore, these events are not relevant to the 1800s, solidifying the Civil War as the correct answer for this question.