Brother Number One

It is 1975 and Khmer Rouge troops are forcibly evacuating Phnom Penh's residents to the countrysidean exodus that will ultimately lead to the deaths of hundreds of thousands. Monitoring events from Beijing, an elderly Mao Zedong asks visiting Vietnamese leader Le Duan whether he could ever mount such a merciless purge. Le Duan shakes his head. "No," marvels Mao. "We couldn't do it either."

Two Asian strongmen, dazzled by the moves of the new despot on the block: it's a bizarre moment, courtesy of Philip Short, a gifted biographer who knows his communists. (His acclaimed Mao: A Life...