Although, as said, privateers flew under a flag, it would be a nice touch if you wouldn't be able to see what nation enemy privateers are of, so you could attack another nation without the need of proper war and spread confusion.
Maybe even foment a war between two nations that don't like eachother and are wary of eachother. Send one privateer to harrass one nation, another privateer to harass the other and voila.
But probably just wishful thinking![]()
Well sure things like this did happen, even if privateers had a letter of marque they certainly would over abuse their power. They are pirates after all, just pirates with legal license under the law of the nation they work for, or are a citizen of.

I do know one thing privateers should be able to enter rival territory. At least give us that. Privateers would always go in to gather information about rival nations and do all kinds of unofficial acts for their bosses. Queen Elizabeth used them for information and for other things. She was very shrewd.
