Privateers, under a proper Letter of Marque, are considered to be legitimate mercenaries. No much difference if it is Privateer Babossa of Commodore Norrington who's doing the seizing at sea, as long as the former is operating a letter of marque. Both parties are, basically, part of the same Navy. The other option would be outright piracy.

EDIT: Sure, sovereign states can unilaterally decide whatever they want. in case they are not North Korea they risk to break multiple treaties so. As Russia showed us, one miscalculation in such a sovereign decision bears the risk of turning said country into an isolated country, just like North Korea.