A commission by which the head of a government authorizes a private ship to capture enemy vessels.
The authority to do such capturing is granted to private vessels in letters of marque and reprisal. In the technical sense, a letter of marque is permission to cross over the frontier into another country's territory in order to take a ship; a letter of reprisal authorizes taking the captured vessel to the home port of the capturer.
Since letters of marque and Reprisal allowed privately owned and operated vessels to carry out acts of war, the practice came to be known as privateering. Privateering was frequently encouraged from the period between 1692 to 1814, at which time weaker countries used privateers to hurt a stronger country in the way guerrilla warfare is currently used. Privateers operated concomitant to regular navies. Their main purpose was to annoy the enemy; however, an enemy's merchant vessels were often seized in retaliation for acts of hostility.
The system of privateering was subject to extensive abuses. In the absence of proper letters, a privateer was tantamount to a pirate. Piracy is subject to severe punishment throughout the world. Although privateers allegedly existed in order to support the defense of their sovereigns, they frequently acquired much personal wealth through their activities. In addition, since privateers were not subject to the same discipline as a regular navy, they yielded to the temptation to seize ships beyond the scope of their authority.
Such abuses, and new theories of naval warfare led civilized nations, in 1856, to sign an agreement outlawing privateering. The agreement does not prohibit a state from organizing a voluntary navy of private vessels, which are under the dominion and control of the state.
The U.S. Constitution provides that no state can grant letters of marque and reprisal. The federal government is not limited in this right by the Constitution; however, modern custom and treaties prevent it from granting the letters.