I don't see how the cartels are the most powerful in US. The US is where the big revenue comes from. I'm no expert on cartels but obviously their job is to get the drugs over the border and to US cities that they supply. Making them rich and powerful in Mexico. But they're still based in Mexico.
I'd still consider the Mexican Mafia a powerful OC group in California. Power and influence over street gangs within the US.
Why many people consider the Mexican cartels the most powerful in the U.S is simply because they control the distribution and wholesale of drugs in the U.S and the fact they have establish these distribution and wholesaling networks through out the U.S says something. Sure most of their power is based in Mexico but like I said they control the distribution and wholesaling in the U.S, have set up businesses to launder the money back to Mexico, etc. The fact that that a foreign group(s) have managed to do that says something and they maintain via monopoly of the drug trade and intimidation. All of the domestic groups in the U.S are local and at best regional except the Mexican cartels and this includes the Mexican mafia, AB, LCN etc. See where I am getting at?
