In today's NYTimes, Paul Krugman writes
As far as I can tell, America has never fought a war in which mercenaries made up a large part of the armed force, as is the case in Iraq.Similarly, so far as I can tell, America has never educated its children in schools operated by such a large proportion of mercenaries (i.e. for-hire teachers vs. nuns) as it does today, a point made by Milton Friedman in his discussions with General Westmoreland when the US was considering moving toward an all-volunteer army (see Greg Mankiw's earlier posting on this topic).