As the number of missile-wielding states grows and as the nuclear club expands, the likelihood of a missile being unleashed against the American people or their allies—whether by mistake, miscalculation, or a madman—also grows. Missile defense doesn’t eliminate the danger, but it does give us a fighting chance to confront it.

North Korea’s nuclear-weapons tests, missile launches, and threats to attack American territory are hostile acts. What is debatable is how the United States and allies should respond.