"letter of marque" is a correct and usable phrase in written English.
It refers to a government-issued document or commission that authorizes a private ship, known as a privateer, to attack and capture enemy ships without being considered a pirate. An example of using this phrase in a sentence could be: "During the time of war, the captain of the privateer ship proudly displayed his letter of marque from the king, allowing him to legally capture enemy ships and seize their valuable goods."
Dissertation: "Seizing Liberties: Private Right, Public Good, and Letter of Marque in Medieval Marseille". Website.
They brandished an envelope with a Montgomery, Ala., postmark, which they claimed was a letter of marque from Confederate President Jefferson Davis.
A privateer carried a letter of marque from a government, authorizing the privately operated vessel to attack and capture vessels belonging to the government's enemies.
As England, France, Holland and Spain alternately warred and wooed one another in a diplomatic cotillion, you could choose to privateer with an official letter of marque or simply attack everything that sailed.
But at the end of "The Letter of Marque," with Aubrey redeemed in the eyes of the Admiralty and Stephen and Diana happy at last, the series lost much of its volume-to-volume momentum, and what had once seemed to be on the verge of turning into a full-fledged multivolume novel ended up being something rather less ambitious.
In "The Letter of Marque" (1990), for instance, Stephen Maturin and his wayward wife, Diana, reconcile in a scene modeled on the finale of "The Marriage of Figaro," and the reference is not gratuitous: rather, it adds immeasurably to the richness of the novel's emotional texture.
(See Ron Paul on letters of marque, via Politico).
Operating under letters of marque and reprisal, privateers had license to profit by seizing enemy ships and taking them before admiralty courts for condemnation and sale.
Volume 1: Statistical Data of Union and Confederate Ships; Muster Roles of Confederate Government Vessels; Letters of Marque and Reprisals; Confederate Department Investigations.
As president of the Confederacy, the West Point-educated Davis had little use for a navy; his interests ended with privateers and letters of marque.
