I suppose it could be that Trump wants Greenland to be the 51st star on the American flag. I doubt that's the plan though. I don't know what he wants, but I assume it's something more mundane (new SOFA as @Prosuper pointed out, maybe some kind of mineral rights, etc). And when Trump wants to move the needle on something, but he knows he'll run into difficulty, he has a well-worn strategy: If he wants X, he proposes X2. When he knows he's going to face some kind of challenge on an issue, he proposes the most hyperbolic, most extreme version of what he wants. The media melts down and the public melts down. Whoever is on the other side of whatever the issue is, melts down as well. After all of the chaos, Trump backs down, and the other side backs down, they negotiate, and Trump often gets what he wants, or close to it. It's not even all that unique. Developers do it all the time. When they want to put up a new 10 story building, but know they're going to face a bunch of NIMBYs, they go in with plans for 20 stories, take the attacks, and eventually walk it back to the 10 story plan they wanted in the first place. I suspect that's what's happening with Greenland.