What happened to Mexico seems quite clear. Back in the 80s-90s, South American cocaine and marijuana were mainly smuggled via Central America and the Caribbean. Some Mexican marijuana and heroin was smuggled to the US, but quality was low, and there wasn't much money in it.

But then the US clamped down on the Caribbean route. And now cocaine was moving through Mexico. Now most illegal marijuana comes from Canada. So Mexican gangs are mainly moving cocaine (and heroin).

So yes, it's the US drug problem, and the War on Drugs, that's fucked over Mexico.