As far as crazy ideas from Trump go at least this one has some historical precedence, as the United States tried this in 1867 and again by Harry Truman after World War II.
Source: Wall Street Journal
WASHINGTON—President Trump made his name on the world’s most famous island. Now he wants to buy the world’s biggest.
The idea of the U.S. purchasing Greenland has captured the former real-estate developer’s imagination, according to people familiar with the discussions, who said Mr. Trump has, with varying degrees of seriousness, repeatedly expressed interest in buying the ice-covered autonomous Danish territory between the North Atlantic and Arctic oceans.
In meetings, at dinners and in passing conversations, Mr. Trump has asked advisers whether the U.S. can acquire Greenland, listened with interest when they discuss its abundant resources and geopolitical importance and, according to two of the people, has asked his White House counsel to look into the idea.
Some of his advisers have supported the concept, saying it was a good economic play, two of the people said, while others dismissed it as a fleeting fascination that will never come to fruition. It is also unclear how the U.S. would go about acquiring Greenland even if the effort were serious.
Danish politicians were, shall we say, not very receptive to the idea, calling Trump "mad". Via Reuters.
Danish politicians on Friday poured scorn on the idea.
“It has to be an April Fool’s joke. Totally out of season,” former prime minister Lars Lokke Rasmussen said on Twitter.
“If he is truly contemplating this, then this is final proof, that he has gone mad,” foreign affairs spokesman for the Danish People’s Party, Soren Espersen, told broadcaster DR.
“The thought of Denmark selling 50,000 citizens to the United States is completely ridiculous,” he said.
The internet responded.