The War in Iraq lasted 8 years and cost over 1 trillion dollars.
So aside from the immense risk of starting a war with a nation right next door to you, …aside from the fact that there would be no clear strategic objectives, it would also cost a fortune.
If the US wass really serious about supporting the Mexican government against the cartels, then they would start with increasing financial aid to said government. But these very same people would probably be against that idea.
The higher amperage rating is not really relevant - that just indicates the max current that the power supply can provide. If the device pulls less than that, it isn’t a problem.
However, the polarity of the power supply is very relevant. If it’s is the opposite of what the ZX81 needs, and you powered it up, then yes, you may have damaged something.
The polarity should be shown on the power supply - usually it’s a diagram with three circles which illustrate which part of the connector is positive and which part is negative. There is often times one on the piece of equipment as well. Those diagrams need to match.