I just try to think of it like this (and I know this is not a perfect analogy):
What if Russia or China had a beef with either Mexico or Canada and military intervention was a real possibility (if not probable), how would the U.S. feel
A fair analogy. I'm not sure how "the U.S." would feel - perhaps indeed they (meaning, the U.S. government) would behave similar to how Russia is. I can't say.
How I would feel, though, would presumably depend on which party I believed to be in the right. If Mexico had a regime akin to Iran's, with nuclear ambitions, and a history of proxy warfare (and I - like Russia re:Iran! - had had troubles with precisely the type of terror group that they had supported), for crying out loud I would welcome the intervention of any reasonably-sane state!
Russia, of course, cares not for who is in the right, but for which outcome threatens their power (or imagined/aspired power) in the region. That's my point.