One month into Operation Epic Fury against the Islamic Republic of Iran, a long-overdue conversation has finally broken into the open: What, exactly, is the enduring rationale for NATO? For decades, this question has been treated in Washington foreign policy circles as heretical. But it isn't. And to their credit, President Donald Trump and Secretary of State Marco Rubio are now saying so plainly. As Trump recently put it, "They haven't been friends when we needed them. We've never asked them for much. ... It's a one-way street." Rubio has been similarly blunt: "If NATO is just about us defending...