There's a school of thought that we inadvertently create terrorists by interfering in the middle east and thus motivating people to want to do us harm. And by 'we' I mean 'The United States'.
I suspect that a similar argument can be made that the illegal drug trade and gangs are fairly well correlated and that gangs arise because individuals don't derive any security from the police for a variety of reasons. So gangs are the result of police policy, not that police policy is the result of gangs.
I'm not suggesting that this is a 100% solid theory. But there's probably some nugget of truth in it.