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They can demand, of course, but getting a hearing becomes more difficult. Trump appears to have done just that, by threatening that NATO will face a dark future if member states do not help. It can certainly lead one or the other to offer some form of support. On the other hand, it would be difficult to justify that NATO would act as such, if you look at the wording in the treaty that governs the alliance. NATO is a defense pact, but the US is the aggressor in this case. Furthermore, the war is not taking place within the geographical area covered by NATO’s defense clause. Possibly one could count Iranian robots striking inside Turkey, or perhaps attacks on the British bases in Cyprus. But nobody seems to want to do that right now.
