Currently under US Constitution, Congress can still issue Letters of Marque. These letters authorized private merchant ships to pirate enemy ships during times of war. These privateers were used by many powers in the 1600s through 1800s. The practice largely ended with the Crimean War when the Great Powers in Europe signed the Paris Declaration Respecting Maritime Law. The US never signed the declaration but has nevertheless abided by its terms. It would not be clear if the US could effectively agree since letters of marque are in the Constitution so in theory any treaty that goes against the Constitution would be null as far as US law is concerned. But as an aside, many regulators try this type of policy laundering to impose international rules onto US law. Anyways, that is for the lawyers to argue.
The same principle can be seen in future space warfare. Treaties for use of Outer Space have been signed restricting nation-states to use of space for "peaceful purposes." Each nation-state is responsible for "activities of non-governmental entities in outer space, including the Moon and other celestial bodies..." But there may be room for a loophole so-to-speak. The government is responsible for the activities of its private citizens' activities but the nation-state itself (i.e. government) can only use space for peaceful purposes. However, it may be that a nation-state could authorize space privateers by Letters of Marque to fight on behalf of that nation-state in a space war. In such way, that government would be responsible via "authorization and continuing supervision by the appropriate State Party to the Treaty." But that government itself would be using space for peaceful purposes, wink wink smile :). So one day we may have space privateers and mercenaries fighting proxy wars in space as part of on-Earth conflicts. In a farther future, privateers could be authorized (I use this term loosely) to take over asteroids being mined by enemy-flagged mining rigs or privateers invading and taking over whole colonies. A nation-state could very well exercise 'responsible supervision' by telling its own space mercs to bomb enemy space installations. In a war between 2 space-faring nations, these loophole violations of the Treaty would most likely be conveniently ignored. In the case of an Earth nation conflict against an independent or break-away space-colony "nation" (presumably not a willing party to the Treaty), such violations would certainly be ignored as space (interplanetary) warfare would be the primary way in which such a war would be prosecuted.
