Few contest that antisemitism—history's oldest hatred of a religious and ethnic group—has had an unmatched post-Holocaust resurgence. In the U.S., Canada, and across Europe, the number of antisemitic attacks broke records for each of the past two years. Jews are 2.5 times more likely to be a victim of a hate crime than blacks, and twice as likely as Muslims. There has been plenty to feed the hatred. Egregious old tropes, such as the notion that Jews are only loyal to themselves and control the world's finances, persist. Meanwhile, social media has spread at lightning speed a litany of new...