Today in 1815, the United States of America&—with the aid of Jean Lafitte's pirates, Choctaw and liberated Haitian slaves—defeated the British in the Battle of New Orleans, the final battle of the War of 1812. I may be wrong, but I think the War of 1812 was the only US war to've started and ended unnecessarily (in formal terms). It started, in part, over a dispute whose settlement didn't reach the US in time to stop us from declaring war, and ended with a battle fought two weeks after the peace treaty, the Treaty of Ghent, was signed. (But according to Wikipedia, ratifications had yet to be exchanged, so maybe the war wasn't quite "over" when the Battle of New Orleans was fought.)
BONUS: And on another, unrelated note, it's Elvis' birthday.