Two suspects have been arrested and charged with throwing bombs — the prevailing term is “improvised explosive devices” — on a crowded New York street. The act of terrorism is an ominous watershed for the city and the nation. It is vital that such violence be stamped out by being punished as fully as possible so that our society does not become accustomed to violence mixed into legitimate political debate. We must not tolerate terrorism on our streets. The mayor of America’s largest city must show that he understands what is at stake. But Mayor Zohran Mamdani has failed that...