A
letter of marque was an official letter from a government
authorizing a private individual to raid and capture merchant shipping of an enemy nation.
A
ship operating under a letter of marque was called a
privateer.
The
United States Constitution (Art 1 sec. 8) authorized Congress to issue
letters of marque.
The difference between a privateer and a pirate was a subtle (often invisible) one, and the issuance of letters of marque was banned by international law under the Declaration of Paris[?] in 1854.