At Pioneer Courthouse Square in Portland, Oregon, tens of thousands of people crowded around a massive Douglas fir tree decked with lights. Waiting for the carnage to begin was Mohamed Osman Mohamud, a Somali ‘child’ refugee, who was dreaming of killing them all. In 2010, Mohamud had urged a friend of his traveling to Mecca to pray “that I will be a martyr in the highest chambers of paradise.” He planned to attack Americans “in their own element with their families celebrating the holidays.” And Christmas was the perfect time of the year to do it. While fathers lifted up...