and covertly provide air dominance.

Yeah, I'm sure Vietnam wouldn't mind some US war planes over their heads.....

http://books.google.com/books?id=CsP2lQtoPFoC&dq=fire+in+the+lake&pg=PP1&ots=2uj2v5Ak9X&sig=g0Z9xhvYcxbIQYzKDUtF_rxWa8s&hl=en&sa=X&oi=book_result&resnum=1&ct=result#PPP1,M1

Any time you want to sit down and compare notes on history, just let me know. Most of my reading on Indochina goes back 25 years or so, but I still have some game :)

Our guys in the Vietnam War (mid-1940s to 1975) were in the south. They lost. But the existing regime in Vietnam is the fairly direct successor, albeit modernized and considerably converted to capitalism, of our former enemies. Helping them would not be "been there, done that" at all.

True to its communist heritage, Vietnam is still a remarkably totalitarian state, but it's moving in the right direction. Much of the credit for that, not coincidentally, can be laid jointly at the feet of an unlikely pair of collaborating senators, John McCain and John Kerry, who led the push to normalize U.S. relations with Vietnam during the Clinton administration.

Normalization, renewed ties among families scattered by the post-1975 diaspora, and increasing commercial power and ties notwithstanding, Vietnam is still not remotely as close an ally to the United States as, to pick another relevant example, Georgia. But the point here, if China were to invade, would not be that China is attack an ally of ours so much as that China is using brute military force to subdue and dominate its neighbors.

That is something we ought to discourage and penalize, through a combination of stern, consistent, and very public rhetoric on the one hand, and tangible economic disincentives on the other. China wants not only to be an economic superpower, but a respected one with a seat at all of the same tables that are occupied by the U.S. and E.U. countries. Those institutions aren't for barbarians. When the Chinese act barbarously, the should forfeit those seats, or their progress toward gaining them.

It should go without saying (but since it's fashionable on the Left to falsely assume and assert otherwise with the GOP holding the White House, I'll say) that we ought to do our best to enlist our diplomatic and commercial allies (principally NATO and Australia) in these efforts.

Now I’m sure the United States has plans for all sorts of contingencies including invasions of Canada or Mexico

Really? I would find that unlikely, as the details for any actual invasion will depend so much on the reasons for the invasion, any plan at this point would be pretty well worthless. I can see us documenting communications infrastructure, military bases, radar stations and the like, but nothing approaching an actual plan. War-games scenarios I can see, but nothing that would be meant to be used in operation.

What should the U. S. response be to such an action?

Spend a good long time pouring over those plans to discover not only what China planned to do, by why China planned to do it a certain way. We can probably learn more about their military capabilities by discovering how they planned to handle logistics than by any number of spy satellites.

We could also start talking about selling defensive weapon systems to Vietnam, which would probably be the most punishing thing we can do to China at this point.