Gangs have always existed. Cartels as they are now have not existed for a long time unless your definition of a long time means since the 60's/70's. Violence has always been lowest when there is a dominate cartel. When the government takes down a cartel or its leaders it creates a power vacuum that leads to violence. Thats why the drug war launched in 2006 has caused so much violence. You now have the cartels fighting each other and the government. Considering the stated goal of the 2006 drug war was to lessen violence I think it can only be classified as a miserable failure. You can actually trace a majority of the violence to the arrest of the Miguel Angel Felix Gallardo in 1989.