???
All I can say for certainty is that in the 1970s, I used to write odd letters to random people as a sort of a prank. I wrote to the DEA and asked about fitting out a yacht as a privateer. I got a very nice and thoughtful letter (which I no longer have) telling me the government no longer did that.
Very nice of them.
HEH. 
What other letters did you write?
I’m pretty sure the United States has done this only twice–during the Revolutionary War and the War of 1812. The Confederacy also did it during the Civil War.
Representative Ron Paul proposed reviving letters of marque and reprisal as a weapon in the war on terrorism, but he is outside of the ideological mainstream and his proposal was not acted upon.