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

The Civil War is indeed a significant event that took place in the United States during the 1800s, specifically from 1861 to 1865. This conflict was primarily fought over issues such as states' rights and slavery, leading to a deep divide between the Northern states (the Union) and the Southern states (the Confederacy). The outcome of the Civil War was crucial as it resulted in the preservation of the Union and paved the way for the abolition of slavery through the 13th Amendment.

While the War of 1812 also occurred in the 1800s, it took place earlier (from 1812 to 1815) and is notable for different reasons, such as British interference with American shipping and impressment of American sailors. In contrast, World War I and World War II occurred in the 20th century, making them outside the timeframe of the question. Therefore, the correct response appropriately highlights the Civil War as a defining war fought by the United States in the 1800s.