But the threat of Depopulation Wars is also a reminder that states with a diminishing population are at least as worthy of close attention as those with increasing numbers. In one respect, they may represent a greater danger. This is because they are susceptible to the same condition as any other state, and individual, that experiences a decline and fall: the danger of defeated expectations that have been raised by the prosperity of earlier years but then dashed by the realization that it can no longer play the vigorous role of before. A demographically diminishing state may act aggressively, unleashing the “all-shaking thunder” of King Lear, in an attempt to recapture its lost grandeur: contemporary Russia is the most obvious example.

Perhaps the most important conclusion is that demographics in general merit considerably more attention. Every sort of demographic change—whether an overall increase or decline of numbers, or changes within a particular community—is apt to have wider, international repercussions, affecting the behavior either of that state or of its partners, rivals and adversaries. This means that demographics, not least the complex interplay of rising and falling populations in different parts of the world, needs to be at the very forefront of strategic attention rather than be seen as a peripheral issue.

R. T. Howard is the author, most recently, of Power and Glory: France’s Secret Wars with Britain and America 1945–2016, as well as several other books on defense and international relations.