I was originally going to make a smart ass comment about how Eisenhower first said this, then realized that mercenaries have been around a lot longer than that.
So, I did what I always do and went snooping to find the earliest known opponent of the idea of privatizing the military, when I stumbled upon the following passage from everyone's favorite online encycloptic webtube,
http://en.wikipedia.org/wiki/Mercenary#Laws_of_war">Wikip*dia:
The Protocol Additional GC 1977 (APGC77) provides the most widely accepted international definition of a mercenary, though not endorsed by some countries, including the United States. The Protocol Additional to the Geneva Conventions of 12 August 1949, and relating to the Protection of Victims of International Armed Conflicts, (Protocol I), 8 June 1977 states:
Art 47. Mercenaries
1. A mercenary shall not have the right to be a combatant or a prisoner of war.
2. A mercenary is any person who:
(a) is specially recruited locally or abroad in order to fight in an armed conflict;
(b) does, in fact, take a direct part in the hostilities;
(c) is motivated to take part in the hostilities essentially by the desire for private gain and, in fact, is promised, by or on behalf of a Party to the conflict, material compensation substantially in excess of that promised or paid to combatants of similar ranks and functions in the armed forces of that Party;
(d) is neither a national of a Party to the conflict nor a resident of territory controlled by a Party to the conflict;
(e) is not a member of the armed forces of a Party to the conflict; and
(f) has not been sent by a State which is not a Party to the conflict on official duty as a member of its armed forces.
All the criteria (a - f) must be met, according to the Geneva Convention, for a combatant to be described as a mercenary.
According to the GC III, a captured soldier must be treated as a lawful combatant and, therefore, as a protected person with prisoner-of-war status until facing a competent tribunal (GC III Art 5). That tribunal, using criteria in APGC77 or some equivalent domestic law, may decide that the soldier is a mercenary. At that juncture, the mercenary soldier becomes an unlawful combatant but still must be "treated with humanity and, in case of trial, shall not be deprived of the rights of fair and regular trial", being still covered by GC IV Art 5. The only possible exception to GC IV Art 5 is when he is a national of the authority imprisoning him, in which case he would not be a mercenary soldier as defined in APGC77 Art 47.d.
If, after a regular trial, a captured soldier is found to be a mercenary, then he can expect treatment as a common criminal and may face execution. As mercenary soldiers may not qualify as PoWs, they cannot expect repatriation at war's end.
Does that mean Blackwater mercenaries can be treated like common criminals and face execution for their crimes?