A privateer or corsair used similar methods to a pirate, but acted while in possession of a commission or letter of marque from a government or monarch authorizing the capture of merchant ships belonging to an enemy nation. For example, the United States Constitution of 1787 specifically authorized Congress to issue letters of marque and reprisal.
--Piracy (via Wikipedia)
There was an absolutely fascinating article in Harpers that I just read about the debatable morality of state-sponsored privateers during the Revolutionary War in the United States, and its parallels to today's crazy financial markets where privateers plunder our retirement funds instead of British supply vessels.But the article, By the rivers of Babylon by Lewis H. Lapham, is entirely unavailable in whole or in part on the Internet... so it's impossible for me to quote it, short of typing it in myself. Harper's Magazine may be 158 years old, so I guess they're a little bit behind the times on this "Internets" thing...
UPDATE: Check it out on Gary's posterous, it's now online.