Trump says things like this at the beginning of potential negotiations because it puts the other party on their heels and creates uncertainty. There’s about a 0% chance that the United States militarily seizes Greenland. But not taking that off the table makes a lot of of the other options we would like to look at to expand our influence there seem a lot more reasonable to Denmark. Art of the Deal 101. Not saying that any of us should agree with those tactics, but that’s exactly what they are. A lot of bluster.