This will be a brief post in response to the gangs in the military topic below.
What images do you typically associate with inner city gangs? Poverty, drugs, illegal weapons? Probably all of the above. It is difficult for gangs to annihilate each other because, when it comes down to it, they aren't very efficient in the use of force. Routinely, homes and cars are shot up with a bazillion rounds by people who don't really know how to fire a weapon (i.e. the ubiquitous side-cocked .45). Also, street fights get pretty bloody and nasty, and while I'm not saying some of those people don't pick up good fighting skills, they rely mostly on numbers and intimidation.
Watch this video for a good example: Latino Gang Fight . These kids are throwing wide, telegraphed punches, stopping every two seconds to pull up their pants, are very unsure what a headlock is supposed to look like, etc.
Compared to the skilled grappler at the beginning of this video : Army Combatives exercise
These fights are over much quicker because your opponent is incapacitated by broken bones, unconsciousness. Less time for the cops to show up.
My point should be obvious. Gang members enlist in the military, especially in the reserve forces to receive free weapons training, fighting skills, and to get in good physical shape. They return to their communities to help train other gang members. They become more efficient in the use of violence, their shoot-ups aren't as noisy and easy to catch. A 'hit' before would take a car load, where as only two infantry soldiers can handle the thing much better.
Also, gangs spread their ideology world-wide through military deployments. They can take advantage of military vehicles for transport. All of this is completely free to them. CID conducts assessments periodically of gang activity. Here is a censored version of the 2004-2005 assessment: US Army Criminal Investigation Command Gang Activity Assessment