US use of "soldiers of fortune"

[Evil_Maniac From Mars]

Evil_Maniac From Mars, so... price before morale and laws, huh?

Answered with:

Now, as they don't belong to the official army, they do not have to follow the laws of war set by the Geneva convention.

So, what laws? Are there laws saying that mercenaries cannot be hired by the state? If so, I would be much obliged if you could point them out. There is nothing wrong with mercenaries as long as they are efficient, disciplined, and cost-effective.