In a tour de force, prize-winning New York Times reporter C.J. Chivers traces the invention of the assault rifle, following the miniaturization of rapid-fire arms from the American Civil War, through WWI, Vietnam, to present day Afghanistan when Kalashnikovs and their knock-offs number as many as 100 million, one for every seventy persons on earth.

At a secret arms-design contest in Stalin's Soviet Union, army technicians submitted a stubby rifle with a curved magazine. Dubbed the AK-47, it was selected as the Eastern Bloc's standard arm. Scoffed at in the Pentagon as crude and...