The two biggest things we could do to curb the cartels in Mexico there is absolutely no political appetite for.
First we need a real federal gun registry database to start drying up the influx of weapons. Almost every gun in Mexico came from the US, almost half of the trace requests come up blank because these forms are all filed on paper in a warehouse that they aren't even allowed to index properly. The most nauseating thing is the gun Industry is in on it... don't believe me? Look up (38 Super El Presidente model).
Second we need to legalize drugs. You can make it so it needs to be bought through a doctor and or psychiatrist, but make all that shit pennies on the dollar. Weed revenue might have dried up, but a significant portion of the rest of their revenue comes from harder drugs.