A letter of marque is a formal document which issued by a government to a citizen to allow him or her to seize goods or citizens of other nations. These letters have historically been used as political tools, both to allow people to deal with private disputes and to engage with another nation without openly declaring war. Some countries still allow letters of marque to be issued, although most do not use the privilege; in other nations, they are no longer legal. The United States is one nation in which they are permitted, under the terms of the Constitution.
Essentially, a letter of marque creates a legal pirate. A citizen with such a letter was historically allowed to outfit a ship for the purpose of piracy, taking that ship beyond national waters and attacking enemy shipping. Attacks on a nation's merchant marine can be very powerful politically, and the holder of the letter can sell any goods confiscated, while captured citizens could be used as sources of information or bargaining chips. One famous holder of a letter of marque was Sir Francis Drake, a British Vice Admiral who attacked Spanish shipping.
A ship outfitted under a letter of marque was sometimes known as a private man-o-war, in a reference to the fact that it was used to supplement a national navy. Such ships were also known slangily as privateers, as were their captains and crews. In France, where these letters were also known as lettres de course, these ships were called corsairs. Most nations discerned between privateers and pirates; as privateers operated with the legal blessings of their government.
Initially, letters of marque were designed to allow people to redress personal wrongs. Marque in Old French meant “seizure” or “reprisal.” For example, the property of a French merchant might be stolen in Italy, and the merchant would apply for a letter of marque which would allow him or her to intercept an Italian merchant's goods to make up for the loss. Many governments respected these letters because they wanted to be able to use them as tools themselves.
Over time, politicians began to realize the potential of a government warrant which would allow someone to confiscate goods from citizens of another nation. A ship using a letter of marque would be expected to outfit itself, and as a gesture of appreciation for the letter, it would surrender some of the confiscated goods to the government. Governments could therefore ensure that foreign shipping would be interrupted without having to bear the financial burden, and they would often come out ahead.