I want to buy your house.
No, I really want to buy your house – it would be good for me.
I’ll pay a fair price for it – about £1,000 should cover it.
What do you mean, it’s your house and you don’t want to sell it to anybody, let alone me? The nerve of you! I want it, and I’m going to get it.
I’ll just take it off you if you don’t sell it to me. I’ll get all my friends and we’ll fight you and then take the house off you if you don’t sell it to me for £600 right now.
Trump threatening to take Greenland is not about Greenland. Such questions miss the point of Trump’s threats. It doesn’t really matter how he would go about “taking Greenland”.
What Trump is hoping to achieve with such threats is not to take Greenland or to make Canada the 51st state, but to disband NATO with the other NATO members’ own hands. Because Trump’s hands are tied by US Congress approval. He will not get a 2/3rds majority to pass any law repealing the application of the North-Atlantic treaty in US legislation.
So the only way he can do it is by antagonising the other NATO members so much – for example by making territorial threats – that eventually they will “get the message” and declare NATO dead themselves. Or they will have to act as if NATO does not really matter anymore, it’s just words on paper if the US ally behaves like an enemy.

Watershed — the melting Sermeq glacier, south of Nuuk, Greenland
You can’t sue the US for not meeting its obligations under this treaty, as the treaty itself does not stipulate any specific obligations in terms of defence contributions. How the NATO treaty gets enforced is up to its members and its power so far rested on the willingness of its members to act as one to maintain its credibility. The famous article 5 of NATO only specifies that a member “will assist the Party or Parties so attacked by taking […] such action as it deems necessary”.
If its members reach such a situation that the treaty is unenforced law, then it’s just as good as void.
So far the Danish strategy is to have it fizzle out; at the moment Trump is more occupied with Gaza (prime real estate, the Riviera of the Mediterranean!) than with that cold place. The Danes won’t sell it; if Trump invades Greenland (hilarious; it’s more likely he wins a pro-golf tournament) Denmark will invoke Art. 5, and all European NATO countries will come to aid (maybe not Hungary, they’re just a Russian province).

A blockade of any type is pointless. Canada has a 4000 km sea border with Greenland, the US none (zero, nada, nothing). The US import tax on Canadian goods is interpreted as the ultimate back stabbing move, and anywhere where Canada can obstruct the US they’ll do it (at the moment there’s booing during the US national anthem, and regular shops have removed US products from the shelves)
So far Europe has played it like “let’s give him something he already has or Europe planned to do, but that he believes is his win”. However, if he continues (or the US in the future) there are already voices in Brussels and European think tanks to drop the US as a partner and shift our attention to China.
Here an impression of what G7-leaders feel when explaining Trump facts of life: total despair

So, yes, Denmark will refuse to sell Greenland. And no, there’s nothing Trump can do about it
