But Trump hasn't actually done anything. Issued dumb threats? Sure.

That's not exactly true. He did jack up tariffs over this. He's not just laying back and saying, "I'd sure like to have Greenland someday" - he imposed a tariff on eight countries because they have not acceded to his demands. It basically scuttled the US-EU trade deal, which was pretty close to being done.

Plus, issuing dumb threats is doing something. Threats matter. Because other nations have to act on the threats. They can't assume they're nothing - partially because every national security threat has to be taken seriously and prepared for, and partially because Trump in particular has demonstrated that he's willing to do things that might otherwise have been dismissed as exceedingly unlikely.

And that's especially the case in a mutual collective security alliance like NATO, where the entire premise of the arrangement rests on the good-faith assertion that the parties will act in their common collective defense. If one country within the alliance issues threats against another country in the alliance, dumb or not, it reduces the effectiveness of the alliance.

They've asked him nicely to stop making the dumb threats. He keeps making the dumb threats. So they're going to start taking measures in response to the dumb threats.