What has he done (as of opposed to said) to damage NATO? I agree that he had said a lot of stupid (and perhaps damaging) things about NATO.
You missed the boat here, IMO.
Trump says these things precisely. It’s his art of the deal. He wants everyone to know that agreements are vulnerable and that nobody should take advantage of the USA because they risk losing a deal that generally benefits them more than us. He’ll even break up a deal or two to prove that no deal it should be taken for granted, thus giving the USA much more leverage across the spectrum.
Yes, people hate him for this. “That’s no way to treat our allies! We should be like Obama, and give in and let them take advantage of us, as long as we don’t step on their toes.” Perhaps that’s right, that unity is better than results. But to imply that Trump’s rhetoric about NATO was not part of a strategy is not an accurate depiction, IMO.