Here's some evidence of what letters of marque were used for, ican:
Quote: The procedure for issuing Letters of Marque and the issuing authority varied by time and circumstance. In Colonial America for instance, colonial governors issued them in the name of the King. During the American Revolution first the state legislatures, then both the states and the Continental Congress, then after ratification of the Constitution, Congress authorized and the President signed Letters of Marque. A ship owner would send in an application stating the name, description, tonnage and force (armaments) of the vessel, the name and residence of the owner, the intended number of crew, and tendered a bond promising strict observance of the country's laws and treaties, and of international laws and customs. The commission was granted to the vessel, not to its captain, often for a limited time or specified area, and stated the enemy upon whom attacks were permitted. For instance during the Second Barbary War President James Madison authorized the Salem, Mass. brig Grand Turk to cruise against "Algerine vessels, public or private, goods and effects, of or belonging to the Dey of Algiers".[15] (Interestingly, this particular commission was never put to use as it was issued the same day the treaty was signed ending the U.S. involvement in the war—July 3, 1815.[16])
A Letter of Marque and Reprisal in effect converted a private merchant vessel into a naval auxiliary. A commissioned privateer enjoyed the protection of the laws of war. If captured, the crew was entitled to honorable treatment as prisoners of war, while without the license they were deemed mere pirates "at war with all the world," criminals who were properly hanged.[17]
For this reason enterprising maritime raiders commonly took advantage of "flag of convenience" Letters of Marque, shopping for cooperative governments to license and legitimize their depredations. French/Irishman Luke Ryan and his lieutenants in just over two years commanded six vessels under the flags of three different nations and on opposite sides in the same war.[18] Likewise the notorious Lafitte brothers in New Orleans cruised under letters of marque secured by bribery from corrupt officials of tenuous Central American governments, or the briefly sovereign nation of Texas, to cloak plunder with a thin veil of legality
that's from "Letters of Marque" in Wikipedia. Since you're a conservative activist, in the sense that you feel you're not bound by the original intent of the fouunding fathers but insist on reinterpreting what they clearly stated in furtherance of your conservative revisionist outlook, you can try to reinterpret what Blackwater did as coming under that rubric, but whatever jjustification the Bush administration thought they had, it was clearly not a Letter of Marque. Blackwater was not a ship. The Bush administration seems to have acted unconstitutionally. No surprise there.