As Barack Obama returns from Europe on Tuesday, he has in bright, bold strokes revealed his signature on the world stage: He is Obama the rationalist. A diverse set of Obama decisions in recent days have a common theme: a leader who sees himself building a more orderly, humane world by vanquishing outdated thinking and corrupting ideology. With a rapid series of major announcements and rhetorical gestures, the new president has done more than turn from Bush-era policies. He has shined a vivid light on his philosophical outlook on the world — and how starkly he differs from his predecessor on...