Labor was scarce, a crucial fact in a society built on the muscle power of animals and humans. Then, after a period of warming, beginning in about 800, a longer cooling set in around 1315, provoking famine and pandemics, and lasting for several centuries. Medieval societies tended to militarize as they struggled to respond and adapt, he notes.

As he describes it, a “Late Antique Little Ice Age” that cooled the Earth from around 540 to 660 shattered classical systems and gave rise to smaller, more localized forms of government.

Morillo, a historian at Wabash College, examines warfare in the epoch before the industrial era and emerges with a surprising message: The medieval era was shaped and reshaped by climate change. In our emerging information age, the computer chip promises to be the foundation of military power, as contemporary militaries strive to collect oceans of data, process it and act on it before the adversary does. Then, during the industrial age, machinery became key to combat - tanks, airplanes, steel warships and locomotives. Indeed, the Mongols, the military superpower of the Middle Ages, as it is put by Stephen Morillo, the author of WAR AND CONFLICT IN THE MIDDLE AGES (Polity, 257 pp., paperback, $26.95), may have controlled more than half the horses on the planet. In most agrarian cultures, the horse was the basic unit of power.

Military historians have long taught that how a society fights is linked to how it produces.

War horse scholastic