The US has invaded Mexico before (and taken half of its land in the subsequent treaty). Mexico is also substantially under US domination since (along with much of Latin America), it is just that our domination has been softer and maybe more beneficial. The situation is certainly not the same, but there are significant risks of very similar border conflicts arising in the coming 2-3 decades, and the US will absolutely respond in as dominant a way as it can. The US would absolutely not tolerate a border country joining a binding oppositional alliance. I don't think the analogy is so bad as you make it out to be and I think the US would try a series of actions to diffuse the situation similar to what we have seen in this conflict (but the US would probably be more capable and more likely to avoid the final escalation).