The US Constitution authorizes the federal government to grant letters of marque- license for private parties to seize enemy ships for profit during wartime; a common practice until the latter nineteenth century when most nations agreed to sign a treaty banning it. My question is, were letters of marque ever intended or used to grant a similar license to mercenaries acting solely on land? I vaguely recall that the leaders of partisans and other irregulars were sometimes commissioned as officers but I couldn’t give you a cite. And anyway I’m more curious about actual mercenaries: private parties who fought solely for pay or permission to plunder. Did any such bands ever have such a legal defense against being accused of brigandage?
Although you ask about mercenaries “acting solely on land,” it’s worth mentioning that many privateers did much of their raiding on land. For example, Henry Morgan attacked Granada in Nicaragua, Principe in Cuba, and Panama City in Panama by marching overland more than 40 miles from where he landed on the coasts.
I don’t know, but suddenly I want an E-mail of Marque.
I do know that mercenary armies were often employed by sovereign countries, although I don’t know if they ever received any sort of Letters of Marque. My assumption is that they were paid by the sovereign and probably had some sort of claim to spoils.
Swiss mercenaries were a big deal back in the day: Swiss mercenaries - Wikipedia
Would you settle for a Text of Marque?
I’m not sure why a mercenary would need the equivalent of a letter of marque. He’s hired by the king (for instance), follow his orders, walks behind his colours, and is part of his army.
The lettre de marque essentially allowed to tell apart privateers (whose activities were lawful) and pirates (who were criminals). I see no much need for that on land. I assume that if a company of unemployed mercenaries is rampaging around, it would be relatively easy to figure that out.
Finally, I suppose that the captain of the company would have signed a contract of some sort, and that if it had come to that, he could had provided it.
I think that both the legal niceties and the strength of a letter of marque as a defense in court could be overestimated. In other words, if Henry Morgan had fallen into Spanish hands he would have been hanged as a pirate (or burned as a heretic), not held as an honorable prisoner until exchanged–more or less the fate of Lord Cochrane, for instance.
The Laws of War had a lot of grey areas during the early modern period for situations of this kind. Still do, of course.
There, of course, were times during the European wars when there were various bodies of men wandering around killing and pillaging, and determining if they were led by a Prince or a pauper might be difficult and was surely regarded as irrelevant. If they lost a battle they were ransomed or hung regardless.
It would often depend on the state of relations between the countries involved. For example Henry Morgan’s raid on Panama took place after the signing of a peace treaty between England and Spain. However, the treaty (as was common at the time) had a grace period for actions undertaken within six months after its signing. Hearing that a treaty was imminent, Morgan had deliberately put to sea so he could plead ignorance that one was in effect during his Panama raid.
The British government was well enough pleased with Morgan’s actions, but to appease Spain Morgan was forced to return to England to answer charges of violating the treaty. He convinced them that he was unaware of the treaty, and ended up being knighted and made governor of Jamaica.
Captain Kidd on the other hand claimed to be operating as a privateer, but was executed in part due to political and business considerations.
I see it as a question of numbers.
A Letter of Marque on land isn’t really a necessity. Organized land armies all wore uniforms–even the mercenaries–to ensure command and control over forces. Brits wore red, Americans wore specified colors, Hessians wore yellow (IIRC), etc. Militia, or unorganized were rag-tag variations back then, but were assumed to be anti-British, or at least identifiable by their actions (“Hey, they’re shooting at us! Shoot back!”). Due to the vast numbers of soldiers on the field (thousands of individual fighting units coalesced into one huge dynamic body), and a requirement to know who your folks are and what they’re doing, custom dictated they wore a uniform. That acted as their default identification.
But you don’t have that luxury of identification in the Age of Sail. You have to get within vocal range of another ship to speak with it’s Captain, even if you gotta send a launch from yours to theirs. Conversely, you’re in firing range when it’s too late. Taking the rag-tag ‘militia’ appearance of a privateer as a disguise, you need some way to verify their identification; that’s where that Letter comes in. ‘We look like a whaling ship, but our letter here authorizes us to be Americans in disguise, because we’re a single fighting unit (one ship).’
Also, communication at sea was far, far slower than that on land. A single ship could be out at sea for months, and would be away from it’s commissioning authority for that length of time. That Letter would be the sole authorization for it’s operations. A land army? They could send a rider back to it’s higher headquarters or civilian authority for instruction and concurrence in operation.
And now that I’ve typed all that out, reasoning it out to myself, I think my second point (slow communication) is probably the more practical reason.
Tripler
I’m slow, but I get there. 
“U R free 2 plndr brit shpng wherever LOL”, followed by a .jpg of a fancy wax seal ?
I strongly suspect that LoMs were just as much of a legal defence against convenient trials for piracy coming from your *own *side anyway ; or from accusations coming from the very people who gave you the letter of marque should peace be signed and your head become a tempting good will token. Insurance against being thrown under the bus, as it were.