I think a closer equivalent would be if Russia lobbied the Mexican government to join them in a joint self-defense pact and leaked that they were going to be putting bases near our southern border. This leads to the US invading Mexico to establish a buffer zone to prevent incursion into our territory. In that scenario, would we expect Russia to come to Mexico’s aid and funnel military aid and weapons to them to fight us even though no pact was signed? Perhaps they would, but SHOULD they?
This seems to be, by most accounts I have seen, closer to what happened with Russia/Ukraine.
Your hypothetical is wrong. A more accurate scenario would be some part of Arizona decides to join Mexico. The state of Arizona says no, you can't do that, and a small civil war ensues. Then the Mexican army starts sending arms and soldiers to fight and they manage to capture about half of Arizona, and some of New Mexico.
Everybody negotiates a cease fire, and except for some shooting across the ceasefire line it is generally peaceful.
And then the Mexicans announce their plans to invade, and they invade Texas.
Should the rest of us in other parts of the USA help Texas?
Why is it when you guys trot out this trope it’s always Canada and Mexico you mention and never the more likely scenario of Cuba, Nicaragua, or Venezuela?
include the part about russia formenting a coup in mexico, and that seems to sum it up. throw in biolabs for whatever reason, and russia actually running air defence assets, MLRS systems and who knows what else, some of it apparently from day 1, and bragging about it to their own press.
I think canada is a better analogy for this then mexico, also. much closer to us culturally and linguistically.