William Walker may have been one of the 19th century's most brilliant minds. He graduated from the University of Tennessee in 1838, at the tender young age of 14, then went to medical school, law school and briefly served as editor of the New Orleans Crescent, all before he was 25. He also happened to be extremely proud and aggressive -- a winning combination that saw got him wounded in two duels as a young man -- and, unfortunately, a big racist. This latter trait surely contributed to the development of what would be his lasting legacy: the filibuster.
We don't mean the legislative stall tactic used by long-winded politicians; back then, "filibuster" meant "a private individual who engages in unauthorized warfare against a foreign country, often with the intent of overthrowing the existing government." In Walker's case, it was Latin America -- specifically Nicaragua, though his ambitions weren't limited to that country. Wikipedia:
"In 1853, he unsuccessfully attempted to stage an insurrection in the Mexican states of Sonora and Baja California. Later, when a path through Lake Nicaragua was being considered as the possible site of a canal through Central America, he was hired as a mercenary by one of the factions in a civil war in Nicaragua. In 1856 he declared himself commander of the country's army and soon after President of the Republic. After attempting to take control of the rest of Central America and receiving no support from the U.S. government, he was defeated and eventually executed by the local authorities he tried to overthrow."
Almost as fascinating (and frightening) as what Walker did was what he tried to do. Convincing many Southerners of the desirability of creating a slave-holding empire in tropical Latin America, he enjoyed great popularity in the South, where he was referred to as "the grey-eyed man of destiny" and "General Walker." ("Destiny," after all, was a hot-button word when it came to American expansionism.) He died before he could capitalize on much of that political power, just a few months before the nation was convulsed by the Civil War.
For more on this fascinating but little-known figure from American history, check out this Wikipedia article, this movie starring Ed Harris as Walker, or this movie in which he's played by none other than Marlon Brando.