Trump is "strengthening NATO" by telling the other members to pull their own weight, or the US will walk away. That may in fact strengthen NATO, if the other members do what Trump wants them to. If they don't, though, it could destroy NATO.

My impression is that Trump is negotiating completely differently than any previous US president. He's throwing extreme positions out there as a starting point, then trying to negotiate back to something reasonable. (Much of the outrage in the press is directed at his opening statements - both content and style - not at the end position.) This approach may work, if others adjust to his style. But international diplomacy is not like business negotiations, and he may wind up alienating countries that he really needs to keep as friends.