A letter of marque and reprisal was a government-issued license authorizing a ship or an individual to attack and seize enemy ships in wartime. Sailors under the protection of such a document were referred to as privateers.
These letters were taken advantage of by merchants, entrepreneurs, and even former pirates. They essentially allowed their bearers to engage in legal piracy against strictly-defined targets — specifically, the ships and towns belonging to any nations at war with the letter’s issuer. This protected privateers from being tried for piracy in their home nation, and was also intended to afford them protection under the laws of war if captured by their enemy. Of course, enemy nations sometimes would ignore this, and treat captured privateers as pirates despite their letter of marque.
A letter of marque could usually be simply purchased, and sometimes they were even given out for free. They sometimes were issued to pirates in conjunction with a royal pardon — in effect, ending their piracy against the issuing nation while gaining that nation new manpower to help fight their wars. These documents were effective ways to maintain an operating military force in the colonial regions, without having to pay to maintain a regular army or naval force. They were attractive options for all involved. The issuing nation gained firepower without any of the usual costs of making war, and could earn revenue from the letters’ sales. Those obtaining a letter of marque were free to seek a fortune in plunder under protection from the hangman’s noose.
The practice of issuing letters of marque died down at the end of the eighteenth century. The Paris Declaration of 1856 outlawed the practice of privateering, and was signed by seven major European nations, including Britain and France. But the United States continued issuing these documents, citing its relative military weakness as a factor biasing the declaration in favor of the strong naval powers. Modern wars saw a move towards larger standing armies and navies, making the practice of privateering essentially obsolete.
While the United States ceased issuing letters of marque in 1815, there were some calls for a return to the practice. During World War II, several civilian blimps were used as privateers to hunt down German submarines, but they were never officially given documents. At the start of the War on Terror in 2001, calls were made in Congress to reinstitute the documents to authorize citizens to target terrorists, but these attempts gained no traction.