For an interesting account of gang activity in the U.S. military, read this account. It would, of course, be absurd to blame all U.S. atrocities in Iraq on the presence of members of black and Latino gangs and the Aryan Nation. That said, I suspect military recruiters are quite happy to have young men with this sort of background help do the dirty work of"pacifying" the local population and even win a Purple Heart or two in the process.
Sociological footnote. We should avoid demonizinggangs in civilian life. For example, consider the gulf between the reputation and the reality of the Hells Angels. Bottom line: Lurid accounts of gang activities very often provide an excuse to extend police surveillance and powers.
Hat tip to Lew Rockwell over at his eminently readable blog.