While the US is busy decaying, it has a lot less time and resources to fuck with its favorite neighbors.
Meanwhile, Mexico is dealing with several major internal and external pressures that culminate in El Pacto de Plata. This reduces the existential threat of the Cartels, but it's an uneasy alliance. Mexico needs a way to relieve the pressure of dropping drug prices, which the Cartels are expressing through massacres and near-enslaving of vulnerable populations. La Sin's ranks are massively swelled with Central American Climate Refugees, so a reinvasion of Nueva Zapata would be too unpopular to pursue. But the US is on the verge of collapse to the North, and its military is ensnared halfway across the globe. So the Cartel's armies were sent to Second Mexican-American War ("La Reconquista"), allowed to keep whatever territory they seized under a Protectorate.
Enthusiasm for this invasion was high:
- It is silly to think that the national character of Mexico should not hold a grudge for the US's aggression in the Mexican-American war;
- The US is on its knees at this point, and has many natural resources in the West;
- Refugee influxes massively increased as Central Americans fled climate change into Mexico. War was a natural way to make use of the additional population and have them earn their citizenship.
The US was militarily overextended and dealing with internal crises, so the West of the country was simply ceded, creating the new Mexican Protectorate of Hidalgo.
The invasion was messy and involved the far less disciplined members of the cartels, who made incursions far past the desired territory. One massacre of a town that had rallied to resist their incursion led to a racial panic and the formation of the Minneapolis Motor Syndicate.
After the conquest of Hidalgo is solidified, there's a persistent insurgency because particularly frontier Americans are those types, until martial law is established via Vietnam-style massacre of entire towns even suspected of links to insurgents. But the uneasy alliance between the cartels and the Mexican military is extremely unstable, and the two always seem ready for war with each other.