How would the US react if the Russians supported Mexico in joining a military pact against the US, so that the Russians could build military bases and install short range nuclear weapons in Mexico and point then at the US?
This a convoluted scenario, as why would they do this in the first place? The US, as big of an asshole as it is, is not invading Mexico. Mexico is not the least bit worried about it.
Ukraine was very worried about Russia invading them, for years, for legitimate reasons. And what does Russia do to alleviate those fears? Repeatedly threaten them, then actually invade.
A gun happy neighbor you are complicated friends with is very different than a gun happy neighbor who is repeatedly saying they want your house. If the situation afterwards feels unfair, well, that’s Russia’s fault for getting there in the first place.
And if the U.S actually postured itself for invading Mexico, for heavens sake, I would want them to arm themselves to the teeth.