A temporary ban on Muslim immigration? That sounds a little extreme (meaning nobody else has said it recently in public). But is it? Millions of Muslims have moved to Western Europe over the past 50 years, and a sizable number of them still haven’t assimilated. Instead, they remain hostile and sometimes dangerous to the cultures that welcomed them. By any measure, that experiment has failed. What’s our strategy for not repeating it here, especially after San Bernardino—attacks that seemed to come out of nowhere? Invoke American exceptionalism and hope for the best? Before Trump, that was the plan.