I've heard that simply decriminalizing weed in many states has had a material, and in some cases profound negative effect on revenue for mexican drug cartels. Furthermore, many consumers of narcotics are "moving upstream" to prescription drugs. But with less revenue, comes more violence initially as the factions squabble fiercely over a bigger slice of a smaller pie. Eventually this will reach an equilibrium and enough producers will drop out or realize it's not worth it and we'll end up with a few kingpins for the CIA to take out (a la Noreaga, Escobar, et al.).