Dear foreign leaders: I suspect a better response is to roundly mock him for it... Unless there's some other intended effect on local (i.e. non-American) audiences behind the approach.
Giving his crazy talk this kind of sober-statesman response just energizes him and his voter-base because they are motivated to confuse "pissed at an idiot" with "scared of a strong man's challenge."
Let's suppose you're right, and somehow we are stuck with an American President with such a fragile ego that they can be goaded into spending almost all their time and political-capital on a crazy quest to convince Congress, the military, and the citizenry to invade Canada or Greenland.
That's a good outcome. It means the unstable incompetent is kept busy and away from harming Americans in easier ways.
Will something of a conflict start between Europe and the USA within the next few years (with all the IT and economic consequences this would have for european markets...)?
Are we basically left with 3 authoritarian empires in play: USA, Russia, China, each with critical different weaknesses they will want to use against the other?
>Are we basically left with 3 authoritarian empires in play: USA, Russia, China, each with critical different weaknesses they will want to use against the other?
Insert the "it always has been" meme here. Except there aren't 3 authoritarian empires at play. Neither Russia nor China really seem interested in expansionism beyond their immediate sphere of influence (which, to be fair, describes the regional land grabs Trump seems to be threatening) and both are outclassed by the US military by orders of magnitude. It makes people more comfortable to believe they live in a multipolar world, because that implies global stability through force and counterforce, but there is only one authoritarian empire in the world of note and that is the US.
The Western world is just getting a taste of what it's been like for everyone else to deal with the knife-edge of Pax Americana. Just a few idle threats from Trump are enough to scare everyone, wait until the missiles darken your skies because the US has decided to "liberate" your resources from you.
Giving his crazy talk this kind of sober-statesman response just energizes him and his voter-base because they are motivated to confuse "pissed at an idiot" with "scared of a strong man's challenge."