One of the leading causes of death in juveniles in the US is homicide, the statistical majority of which is gang related. At an age where young people should be learning skills to survive in a very competitive world, they're fighting and dying in turf wars. That's very significant.
The primary cause, in my opinion, is lack of parental involvement and poor role models. Gang violence has to be combated at the individual level. Communities can help by providing programs and facilities but the heavy lifting is up to the parents. They should know where their children are and what they're doing 24/7. They should get them involved in team sports, music and academic challenges. Children need a clear system of goals and rewards for meeting them.