President Trump is apparently serious about wanting Greenland for the United States. What once sounded like an opening gambit in a negotiation increasingly looks like a deeply held conviction, one that Mr. Trump has publicly underlined by refusing to rule out the use of force. Europeans are serious about this too, however. Assuming that Europe would quietly accept a takeover of Greenland by the United States and then move on would be a mistake. The recent assertion by President Emmanuel Macron of France that his country would stand in solidarity with Denmark against an attack on Greenland's sovereignty and for the deployment of European troops to the island are only the most recent indications of this growing resolve.