Hi!

It's a somewhat difficult question as there are a lot of concepts that are clear in modern language, but weren't at all clear in the period or the definitions were evolving in the period. This is one such case where "pirate", "privateer" and "letter of marque and reprisal" had no universal understanding and it's use was both colloquial, legal and political, which makes it more confusing.

It's also a pretty long period of time, with evolving rules and a changing actual situation, so i am going to gloss over a lot here and probably be liable to making quite large simplifcations and errors in the process - as i'm not that familiar with all the history here.

I'll first talk a bit about the legal difference between privateering and piracy, then about the consequence of that in how they were treated.

Legal difference between letters of marque and reprisals and piracy

Piracy

There is a long history of some sort of universal jurisdiction and illegality of Piracy. In laymans terms, this means anyone can prosecute (earlier, mostly kill) pirates without that counting as a killing someone elses subjects or going to war.

This has ancient traces, there are extensive roman writings and piracy (against christians) was for example deemed illegal by the Third Lateran Council in 1179.

Later on we have Alberto Gentili (a professor at Oxford) in his Commentaries on the Laws of War (1588). He argues that pirates are not legally entitled to wage war, this means pirate actions are never just under jus ad bellum, rather they are the common enemy of all mankind. Gentili uses this to, for example, argue that an English merchants purchase of pirated goods is nullified, as the pirate never had legal ownership of the goods in the first place.

The legal distinction between pirate and non-pirates was never quite clear. Grotius (1583-1645) , sometimes called the father of modern international law, thinks the law of nations does not apply to pirates, because (akin to what Gentili is arguing) pirates are outside of the jus ad bellum between nations. Thus, pirates can be punished by anyone and their property seized. I won't really go into the semantics of what is and isn't a pirate here, as the usage of the term is quite unclear. Grotius in some writings appears to use the term to enemies that are within the law of nations but in breach of it, where attack on them is also justified.

In later writing, it's quite clear that enemies can also be pirates, so it's not just describing groups that are "outside" of nations, see G. F. Martens writing in 1801 where he says that privateers become pirates by transgressning the rules regulating privateers.

There are old court cases illustrating the view of unviersal punishment for piracy as well, for example Rex v. Dawson (1696) where

The king of England hath not only an empire and sovereignty over the British seas, but an undoubted jurisdiction and power, in concurrency with other princes and states, for the punishment of all piracies and robberies at sea, in the most remote parts of the world

So all in all, pirates were generally a category of international criminals deemed outside some category of legal protection normally afforded to the enemy during rules of war. or by third party countries to combatants in a war, or outside the normal rules for seeking relief for damages. Pirates were also outside the ordinary protection and recognition of property, and not under the protection of prize law.

Privateers and letters of Marque and Reprisals

Firstly we have to look at what marque and reprisals are. A person injured by a another sovereign could seek relief from his own sovereign, who could seek that on his subjects behalf. If the sovereign refused relief, he could allow the subject to seek relief himself, under the law of marque, which is thus a letter of marque. One of the first instances is a portuguese seizure of an English merchant ship, whereby the English crown (1295) gave him a five year authorization to take any portuguese vessel and goods until he had obtained satisfaction. Initially this was probably a way to avoid a war by rather making it a sort of private conflict.

Initially, privateering and letters of marque weren't clearly the same, but the concepts blended into each other over time. Grotius, explains well how the initial concept of seeking relief came to be a general justification for privateering.

The justification for Letters of Marque and reprisals in the age of sail was the general idea of Jus Ad Bellum (literally: right to war, figuratively: just war). Grotius explains it thus in his De Jure Pradae (Commentary on the law of prize and booty) [1]

[A]ll enemy possessions are so many instruments prepared for our destruction; that is to say, through them weapons are provided, armies are maintained, the innocent are striken down. It is no less necessary to take away these possessions, wresting them from the enemy, than it is to wrist the sword from a madman.

Grotius is presuming that the war is just, and as such seizure of enemy property is justified by a mixture of self-defence, reprisals, recovery of costs and prevention of future action against onself. In other words the initial concept of attaining relief for the individual is now viewied on a more national basis.

Grotius is writing generally, but specifically about an incident of Heemsckercks [1] who was under letter (of marque and reprisal) from the prince and the goods he took had to be brought home to the united provinces to be distributed by the local admiralty court. This is typical for privateering, in that there was an established legal procedure for validating the booty to see if it was aquired according to the letter of marque, and to distribute and confirm the new ownership of it. That was a key part of what distinguished it from piracy.

Gritoius developed his arguments more in his later On the Law of War and Peace (1625)

Kings (...) have a Right to exact Punishments (on foreign nationals), not only for Injures committed against themselves, or their Subjects, but likewise, for those which do not particularly concern them, but which are, in any Persons whatsoever, grievous Violations of the Law of Nature or Nations.

Writing later, in 1801, the geman jurist G. F. Martens says that a privateer differs from a pirate in that he (1) is provided with a letter of marque from a sovereign; (2) supposes the state of a war of or reprisals and; (3) observes the rules given to him and attacks only enemy ships or neutral ships with illicit goods.

Continued below