BRUSSELS (Reuters) - U.S. President Donald Trump stands united with other NATO countries in defending all members of the Western military alliance, a senior White House official said on Thursday, after Trump made no mention of NATO’s Article V in a speech. “It’s the core of the alliance,” the official said after Trump gave a speech to fellow NATO leaders in Brussels.  NATO’s founding treaty states that an attack on one ally is an attack on all, but Trump questioned that in his election campaign and has not publicly backed the defense commitment set out in Article V. 