President Obama forcefully assured Estonians—and all NATO’s new allies—that waging war on them meant waging war on the United States. “[T]he defense of Tallinn and Riga and Vilnius is just as important as the defense of Berlin and Paris and London," Obama said. "Article 5 is crystal clear. An attack on one is an attack on all. So if, in such a moment, you ever ask again, who'll come to help, you'll know the answer: the NATO alliance, including the armed forces of the United States of America, right here, present, now." This is the ultimate commitment, given by the ultimate authority, in the very place where the commitment would be tested—and would have to be honored. There’s no turning back from that.