I think the only way to have the Mexican invasion happen is to assume that oil paid for a military build up some time during the late eighties/ early nineties. The price of oil went up during this time giving Mexico the ability to partially recover, and perhaps pay for a rebuilt military. The 1988 presidential elections were very controversial due to a computer failure in the election process and it is possible that President Salinas, who was already deemed a fraud by some could end up assassinated. This would cause chaos and open the door for a national socialist to take power on an anti-American platform.
During the nuclear exchange, I've always assumed it was the Soviet that struck the Mexican refineries. First of all, despite Division Cuba, which seems to be a rushed on the fly commitment, there is no way the Soviets can benefit from Mexican oil. Second there is a good chance that the Soviets already had nukes targeted on Mexico during the actual Cold War. Finally, I just don't see the US nuking a non-nuke neighbor especially given the chain of command chaos occurring within the US.
As for arming Mexico...
MBT: OF-40, an Italian designed and built export MBT. It was similar to, and even used some parts from the Leo 1. In our timeline only the UAE purchased it.
The remaining vehicles could come from a mix of countries but I think they'd primarily be from Italy, France and perhaps Brazil.
Benjamin
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