The US right has a slew of reasons to privatise the US military so rapidly. The most obvious is simple corruption. It funnels money to companies in which they have a huge stake, and who in turn donate a fortune to the Republican Party. This is justified in public by a market fundamentalist conviction that governments can never run anything properly, so their functions must always be sold off.
But this is a secondary motive. The main limit on an aggressive US foreign policy today is the limited number of US citizens who are prepared to kill and die for it. Mercenaries solve the problem: just buy troops in. The public is far less likely to protest against a war if the victims are hardened Colombians in it for the cash, rather than their cousin from Wisconsin who signed up out of patriotism. In mercenary wars, all citizens are asked to give is money, not blood.
http://www.commondreams.org/archive/2007/06/04/1651/
I was reading a book about King John (1166-1216), who was not the snivelling weasel he is traditionally caricatured as, but that's not the point here. The point is, he had the same problem, and the solution then was the same as it is today. The 'proper' way at the time to fight a war was to use the feudal army: every baron owed the king the use of some of his knights, it was the way the system worked. But, the feudal army was slow to mobilise, and tended to piss the barons off, who usually couldn't see the point of all these wars the kings always wanted to fight.
So John was very fond of using armies of mercenaries, which could be mustered much more easily, although this was not without its drawbacks. The first was simply that you couldn't quite be sure of the loyalty of a mercenary army: after all, they're only in it for the readies. But the biggest problem of all was the cost, which in medieval days meant the king must squeeze his subjects by taxing the crap out of them. In the end this pissed the barons off more than waging wars with the feudal army had done.
The main difference now is the national debt; the 'king' can postpone the day of reckoning by fighting his wars on tick...
But just to assure you, there's no shit under the sun that's new. All the problems we face today, we faced in the past too.