A letter of marque is an official warrant or commission from a government authorizing the designated agent to search, seize, or destroy specified assets or personnel belonging to a foreign party which has committed some offense under the laws of nations against the assets or citizens of the issuing nation, and has usually been used to authorize private parties to raid and capture merchant shipping of an enemy nation.
The formal statement of the warrant is to authorize the agent to pass beyond the borders of the nation ("marque" or frontier), and there to search, seize, or destroy an enemy's vessel or fleet. It is considered a retaliatory measure short of a full declaration of war, and, by maintaining a rough proportionality, has been intended to justify the action to other nations, who might otherwise consider it an act of war or piracy. As with a domestic search, arrest, seizure, or death warrant, to be considered lawful it needs to have a certain degree of specificity to ensure that the agent does not exceed one's authority and the intent of the issuing authority.
In the past, a ship operating under a letter of marque and reprisal was privately owned and was called a "private man-of-war" or "privateer." The French sometimes used the term lettre de course for its letters of marque, giving rise to the term corsair.
Letters of Marque were given by France in a very selective manner. Under Napoleon, they covered a six-month period in case a war should come to an end. This meant that captains left port with several Letters of Marque, since expeditions rarely lasted less than a year. Once he returned to harbour, the captain had to hand the letter over to the naval authorities, who destroyed it, creating a greater sense of accountability and rarity.
It is the opinion of Jules Lobel, a prominent Constitutional law scholar at the University of Pittsburgh School of Law, that the origins of this clause and the framers' intent fit well with modern notions of irregular warfare, supporting the view that the President's use of troops in foreign military operations, including covert paramilitary actions, is illegal absent Congress' authorization.
Some scholars, however, view this clause as contemplating action only by private contractors, and not as a limit on presidential power. For example, in 2002, Douglas Kmiec, then dean of the Columbus School of Law at the Catholic University of America, testified before the Senate Judiciary Committee that:
Letters of Marque and Reprisal are grants of authority from Congress to private citizens, not the President. Their purpose is to expressly authorize seizure and forfeiture of goods by such citizens in the context of undeclared hostilities. Without such authorization, the citizen could be treated under international law as a pirate. Occasions where one's citizens undertake hostile activity can often entangle the larger sovereignty, and therefore, it was sensible for Congress to desire to have a regulatory check upon it. Authorizing Congress to moderate or oversee private action, however, says absolutely nothing about the President's responsibilities under the Constitution.
Because the difference between a privateer and a pirate was a subtle (often invisible) one, in 1856 the issuance of Letters of Marque and Reprisal to private parties was banned for signatories of the Declaration of Paris. The United States was not a signatory to that Declaration and is not bound by it. During the 1861-65 American Civil War and the 1898 Spanish-American War, however, the United States issued statements that it would abide by the principles of the Declaration of Paris for the duration of the hostilities. The Confederate States of America issued Letters of Marque and Reprisal during the Civil War. The only vessel to operate under a Letter of Marque issued by the United States Congress since the War of 1812, was the airship Resolute, operated by civilians to patrol the seas for submarines during the Second World War.
The issue of Marque and Reprisal was raised before Congress after the September 11, 2001 attacks, and again on July 21, 2007 by congressman Ron Paul. The attacks were defined as acts of "air piracy," and the Marque and Reprisal Act of 2001 was introduced, which would have granted the president the authority to use Letters of Marque and Reprisal against the specific terrorists, instead of warring against a foreign state. The terrorists were compared to pirates in that they are difficult to fight by traditional military means.