“LOOK AT IT THIS WAY,” Gunnar Heinsohn said. “Your family is in a shooting war with a family across the street. Your forces consist of a father, mother and one child, perhaps two. The other family has a father, mother and seven children, perhaps eight or nine. For your family, the loss of one person would be devastating. The larger family can take casualties and continue fighting.”

We were in London, having coffee before a Jane’s Cityforum conference on “Defense to 2020 and Beyond.” As we talked, generals, staffers, and defense contractors maneuvered among the pastry tables. Heinsohn is director of the Raphael-Lemkin Institute at the University of Bremen and author of Sons and World Power: Terror in the Rise and Fall of Nations, an academic best-seller in Germany. Later in the day he would be lecturing professional soldiers on the “demographic dimension” of future warfare.