Yeah didn't really think that through before writing, majority is the wrong word. I didn't mean attacking the border, I just meant that a lot of the violence occurs near the border as most cartels set up shop in locations most appropriate for later distribution into the U.S.
For one thing, no one's bothering to attack the US border, whereas in Brazil the government is fighting numerous active paramilitary groups which are basically all just fighting each other for control of the drug trade.