When Russia’s Vladimir Putin demanded that the U.S. rule out Ukraine as a future member of the NATO alliance, the U.S. archly replied: NATO has an open-door policy. Any nation, including Ukraine, may apply for membership and be admitted. We’re not changing that. In the Bucharest declaration of 2008, NATO had put Ukraine and Georgia, ever farther east in the Caucasus, on a path to membership in NATO and coverage under Article 5 of the treaty, which declares that an attack on any one member is an attack on all. Unable to get a satisfactory answer to his demand, Putin...