NATO made a formal invitation on Thursday to tiny Montenegro to join the alliance as its 29th member. NATO foreign ministers meeting in Brussels signed the accession protocol under the eyes of the Montenegrin prime minister, Milo Djukanovic. The membership must be approved by all 28 national governments or parliaments, including the United States Senate. When an informal invitation was made to Montenegro in December, Russia responded angrily. Montenegro has a population of about 600,000 and little military capacity, but in December, Adm. Vladimir Komoyedov, the chairman of the Russian Parliament’s defense committee, said, “They are ready to admit even...