Very little was surprising about the conclusion of the special counsel’s investigation. For one thing, it wasn’t surprising that Robert S. Mueller III’s probe prompted great commotion — a federal investigation involving a sitting president is a momentous event, and concluding it, a historic moment. And most, but not all, of the details in the attorney general’s letter of “principal conclusions” were unsurprising as well. Let’s start with question of “collusion.” It was never precisely clear what that nonlegal concept meant. If it means what Mueller reasonably took it to mean — an “agreement,” “tacit or express,” with the Russians...