The Islamic State long ago promised to infiltrate the refugee stream. As far back as November 2015, the Islamic State claimed to have smuggled thousands of jihadists into Europe. It was also recently reported that the Islamic State “planned to infect refugee flows to the West with mass killers, and it has had some violent successes.” The CIA warned that the Islamic State’s “official strategy is to hide its operatives among refugees entering Europe and the United States via human flows out of the Middle East and North Africa.” Western leaders opted not to take these warnings seriously. Leftists have...