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

The correct answer is the Civil War. This conflict, which lasted from 1861 to 1865, was primarily fought over issues such as slavery and states' rights, and it had a profound impact on the United States, leading to significant social, political, and economic changes.

The War of 1812 is also significant as it was fought between the United States and Great Britain from 1812 to 1815. This war was primarily about trade interference and the impressment of American sailors but is less commonly associated with the major conflicts of the 1800s compared to the Civil War.

World War I took place from 1914 to 1918, and the Vietnam War occurred from the late 1950s until 1975, both of which fall outside of the 1800s timeframe. Understanding these conflicts helps illustrate the evolving nature of the United States during its formative years and the complex issues that defined its history.