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

The War of 1812 is the correct choice as it is a conflict that occurred during the 1800s, specifically from 1812 to 1815. This war was fought between the United States and Great Britain, primarily over issues such as trade restrictions, impressment of American sailors into the British Navy, and territorial expansion.

The War of 1812 was significant for the United States as it fostered a sense of national identity and unity, despite various challenges during the conflict. The outcomes of this war also had long-term implications, including bolstering U.S. independence and paving the way for westward expansion.

In contrast, the other conflicts mentioned — the Vietnam War, the Gulf War, and the Korean War — all took place in the 20th century, well after the 1800s. Thus, they do not fit the criteria of being conflicts fought by the United States during that specific century.