Recent events may well provide a preview of this reality. When Xi Jinping announced last year that he was slashing China’s armed forces by 300,000 troops, Beijing spun the news as proof of China’s peaceful intentions. Demographics provide a more compelling explanation. With the number of working-age Chinese men already declining—China’s working-age population shrank by 4.87 million people last year—labor is in short supply. As wages go up, maintaining the world’s largest standing army is becoming prohibitively expensive. Nor is the situation likely to improve: After wages, rising pension costs are the second-biggest cause of increased military spending.

Awakening belatedly to its demographic emergency, China has relaxed its one-child policy, allowing parents to have two children. Demographers expect this reform to make little difference, though. In China, as around the world, various forces, including increasing wages and rising female workforce participation, have, over several decades, left women disinclined to have large families. Indeed, China’s fertility rate began declining well before the coercive one-child restrictions were introduced in 1978. By hastening and amplifying the effects of this decline, the one-child policy is likely to go down as one of history’s great blunders. Single-child households are now the norm in China, and few parents, particularly in urban areas, believe they can afford a second child. Moreover, many men won’t become fathers at all: Under the one-child policy, a preference for sons led to widespread abortion of female fetuses. As a result, by 2020, China is projected to have 30 million more bachelors than single women of similar age.

“It really doesn’t matter what happens now with the fertility rate,” a demographer at the Chinese Academy of Social Sciences told me. “The old people of tomorrow are already here.” She predicted that in another decade or two, the social and fiscal pressures created by aging in China will force what many Chinese find inconceivable for the world’s most populous nation: a mounting need to attract immigrants. “When China is old, though, all the countries we could import workers from will also be old,” she said. “Where are we to get them from? Africa would be the only place, and I can’t imagine that.”

Not so long ago, conventional wisdom in China held that the country’s economy would soon overtake America’s in size, achieving a GDP perhaps double or triple that of the U.S. later this century. As demographic reality sets in, however, some Chinese experts now say that the country’s economic output may never match that of the U.S.

With American Baby Boomers entering retirement, the United States has its own pressing social-safety-net costs. What is often neglected in debates about swelling entitlement spending, however, is how much better America’s position is than other countries’. Once again, numbers tell the story best: By the end of the century, China’s population is projected to dip below 1 billion for the first time since 1980. At the same time, America’s population is expected to hit 450 million. Which is to say, China’s population will go from roughly four and a half times as large as America’s to scarcely more than twice its size.