US President Donald Trump's apparently coercive threat to force Western allies not to oppose his proposed annexation of Greenland or face further damage to their trade with the US, is without both parallel and precedent.

We've had some unusual and unexpected economic threats from President Trump over the past year, but I think it is safe to say this exceeds all of them, and takes us into both surreal and utterly dangerous territory.

If taken at face value, it is a form of economic war being levied by the White House on its closest allies.

That's because it targets allies at incredibly short notice and for a cause that essentially could break up Nato and the western alliance.

This will be leaving officials from those countries absolutely baffled. In fact, it's so outlandish that they may indeed be more baffled than angry.

Nobody in the world would assume that a threat like this - based on acquiring the land of your ally - would ever actually happen. Does Trump really have the backing in the US, in Congress, even in his own administration to do this?

Is this, as some trade officials have to assume, the biggest TACO (Trump Always Chickens Out) of all time? These things can come and go and, economically, these countries have handled the damage so far.

Think of Canada. It has seen its trade with the US slump. But its prime minister Mark Carney's strategy has seen Canada's trade surge with the rest of the world by 14% - which amazingly is worth more than was needed to cover its trade lost with the US.

Carney has been in China this week pushing "a new world order" and pursuing more trade with China, not the detachment sought by some US administration officials.

"This is China versus the world," the Trump administration was trying to persuade the rest of the world just three months ago.

Carney is showing up this approach, something which is perhaps notable background context for the timing of today's intervention.

If, however, we do take Trump's latest threats seriously, they are extremely troubling.

Not so much because of the 10% tariff, but because of its rationale - taking land from an ally, and the act of publicly trying to coerce your allies. How would the world react if China or Russia had sent a threat like this to some of their allies?

The basis of the threat is clearly deeply worrying.

Many in capitals around the world will read Trump's social media announcement and question the functioning of American decision making.

President Trump arrives to meet leaders of the allied countries whose economies he has just threatened at the World Economic Forum on Wednesday.

Most of the world will hope that, by that time, this unparalleled threat will have somehow disappeared.