Keep in mind that Greenland and Canada are both members of NATO, and thus the US would be forced to intervene...
Oh, well...
You mean seriously? Who knows? Both sides have nuclear weapons, don't they? This is not something that ever happened. Would the US armed forces go along with the plan or will there be a coup? Will everybody keep their heads leveled and avoid exchange nuclear explosions even with people dying at home / on the front? Will somebody just put a bullet on Trump's head and stop the madness?
But the thing I can tell is that if you are expecting country lines and military treaties to be immutable, that's not how they work.
"Nuclear explosions are inherently unsafe"
Well, he warns about it.