Marco Rubio gave a speech Saturday to the Munich Security Conference in which he extolled an ideal that's supposedly long out of fashion. "We are part of one civilization: Western civilization," the U.S. secretary of state told his largely European audience. "We are bound to one another by the deepest bonds that nations could share, forged by centuries of shared history, Christian faith, culture, heritage, language, ancestry and the sacrifices our forefathers made together for the common civilization to which we have fallen heir." The speech got, and deserved, a standing ovation. What, exactly, is Western civilization? Americans younger than...