There's a good point made here that "if Russia gained influence in Mexico, that wouldn't justify the US invading it".
But we know that the US would absolutely do that, if it came to it. We know from the Cuban Missile Crisis, we know from the unmatched number of foreign interventions/coups the US has done, we know from the contingency plans that the DoD has had for a long time for invading Mexico and toppling the government, and we know from Trump's threats over the cartels that he'd do it for a lot less than a great power rivalry.
There is a gap between what many countries ethically ought to do, and what they practically can be expected to do. We should be able to tell these apart.