Piracy

Piracy has been a scourge of the seas for millenia, but its definition has become more complex with the evolution of international law. The current primary reference comes from the United Nations Convention on the Law of the Sea, Article 101: " Piracy consists of [inciting, performing, or facilitating]:
 * any illegal acts of violence or detention, or any act of depredation, committed for private ends by the crew or the passengers of a private ship or a private aircraft, and directed:
 * on the high seas, against another ship or aircraft, or against persons or property on board such ship or aircraft;
 * against a ship, aircraft, persons or property in a place outside the jurisdiction of any State;
 * any act of voluntary participation in the operation of a ship or of an aircraft with knowledge of facts making it a pirate ship or aircraft;

Imperial Rome defined pirates simply, in the words of Marcus Tullius Cicero, as hostis humani generis, "enemies of the human race." That designation was an early example of universal jurisdiction; it was accepted that pirates were criminals that had put themselves beyond the law of nations, the only existing source of law. At the time, there was no distinction between territorial and international waters, and no source of international law. The idea of hostis humani generis grew to include other categories, such as slavery and genocide, but jurisdictional issues became more complex.

Today, piracy at sea is a very real problem, but counter-piracy enforcement is complex.

Reprisal and Marque
While the usual phrasing of the framework for privateering is "letter of marque and reprisal", the idea of reprisal, or authorizing national merchant vessels to be armed against pirates, preceded the idea of marque, or using nationally authorized private warships used as part of foreign policy.

Privateering eliminated
In an annex to the Declaration of Paris (1856), which ended the Crimean War, a number of nations agreed to stop issuing letters of marque and reprisal. By that time, standing national navies existed and were more controllable tools of national policy.