Michael Walzer, now 83, is the unanointed dean of the American left. The editor of Dissent magazine for more than 30 years, he has written a book criticizing his comrades’ foreign policy and, subtly but unmistakably, their worldview. He sees both as morally lacking and advocates something more humane and more engaged.
Mr. Walzer is a leftist not for psychological or emotional reasons but for moral ones. He believes, a priori, in practicing human decency: a moral sensibility toward other people and their existences. He thinks...