The Letter of Marque, as was carried by Captain Cornelius Killick in Sharpe's Siege, is a legal document issued by the government of the United States which gave permission to "wage war on enemies of the Republic wherever on the high seas those enemies might be found and extended to Captain Killick the full protection of the said Government of the United States." (Sharpe's Siege).
American Letter of Marque
Such papers offered small protection from men like Horace Bampfylde, who illegally disregarded them, but the papers were necessary for proving the holders were not, in fact, pirates but authorized and commissioned as a private shipowners legally employing their vessels as ships of war.
Since crews were not paid by the state, privateers were entitled to cruise for their own profit. During the American Revolution the Americans found it difficult to form a new navy because over 1,000 letters of marque had been granted to privateers. Privateering continued in the War of 1812 between Great Britain and the United States. The ships of the U.S. Navy numbered in the dozens, while more than 500 vessels were sailing under letters of marque.