From a New York Times story by Peter Baker headlined “Cosmetics Billionaire Convinced Trump That the U.S. Should Buy Greenland”:

One of the odder moments of Donald J. Trump’s presidency came when he publicly floated the idea of buying Greenland. It caused a predictable furor, generated gales of late-night television jokes and soured relations with Denmark, which rejected the idea of selling the giant Arctic territory.

But it was no passing whim. While many assumed at the time that it was just Mr. Trump being Mr. Trump, expressing a far-fetched thought that came into his head, in fact the idea had been planted by one of his billionaire friends and became the subject of months of serious internal study and debate that flabbergasted cabinet secretaries and White House aides.