Canada

We have the support of NATO if it comes to that.

Sadly it's unlikely you'd get any military support. The ranges involved prohibit anything but an armada of aircraft carriers from doing anything significant in a north-american theater of war. So the few NATO aircraft carriers that aren't US owned (UK 2, Italy 2, France 1 (3 helicopter carriers)) won't be able to help you guys out. They (and any troop transports) would be destroyed before they come anywhere close to the US.

It would take many decades for the European half of NATO to develop a military strong enough to take on the US on your side of the atlantic. At that point Canada might be so integrated into the US that the people who'd like us to come help you won't be enough for us to actually risk it.

The only thing we Europeans could realistically do is completely sanction the US. This of course would likely be the excuse the US eventually uses to lash out physically, which could lead to a world war (possibly the last world war). Europe's safest move is to just wait it out until the US snaps out of the insanity, or gets more unhinged and starts attacking itself (civil war. Hopefully a non-nuclear one). Then when the US lies in ruins (or the right people win quickly), Canada might be a country again.

All that said, I think an invasion is very unlikely at the moment. If the order to invade Canada went out tomorrow, much of the military would most likely revolt and depose the government. Of course this might change over time as more loyal generals are installed.

The current administration is filled with morons, but I think even they are smart enough to realize they are not yet in a position to do anything but be economically annoying. So if Trump goes full retard and says he's going to order an invasion, the people around him will stuff him away in an old folks home and let JD take over.