When words lose their original meanings and are repurposed as verbal cudgels, the public sphere becomes a jungle of denunciation, intimidation, and even violence. Right-wing politicians who label all critics of Israel antisemites are the mirror image of those who assume that all Jews are Zionists and all Zionists are racists. One of the many virtues of Mark Mazower’s excellent and timely “On Antisemitism” (Penguin Press) is his effort to restore historical context to a word that has become a generic term of condemnation.