In the decades immediately ahead, East Asia will experience perhaps the modern world’s most dramatic demographic shift. All of the region’s main states—China, Japan, South Korea, and Taiwan—are about to enter into an era of depopulation, in which they will age dramatically and lose millions of people. According to projections from the Population Division of the UN Department of Economic Social Affairs, China’s and Japan’s populations are set to fall by eight percent and 18 percent, respectively, between 2020 and 2050. South Korea’s population is poised to shrink by 12 percent. And Taiwan’s will go down by an estimated eight percent. The U.S. population, by contrast, is on track to increase by 12 percent.

People—human numbers and the potential they embody—are essential to state power. All else being equal, countries with more people have more workers, bigger economies, and a larger pool of potential soldiers. As a result, growing countries find it much easier to augment power and extend influence […]