As Hamas supporters hurl bogus charges against Israel.Last month, Megha Vemuri, president of the Massachusetts Institute of Technology class of 2025, denounced the “genocidal Israeli military” and contended that the MIT community “would never tolerate a genocide.” Such proclamations, widely repeated on Ivy League campuses, invite a look at the actual genocide going on 50 years ago.In April, 1975, troops of the communist Khmer Rouge occupied Cambodia’s capital of Phom Penh. One of the last correspondents to leave was David Aikman of Time magazine, author of “Cambodia: An Experiment in Genocide.” As Aikman recalled:After a few hours, the black-uniformed troops...