At the 2008 summer Olympic Games in Beijing, the New York Times columnist Tom Friedman observed something intriguing about the powerful American team, which won the overall medal count for the games. After wandering through the athletes’ village, he noted, “The Russian team all looks Russian; the African teams all look African; the Chinese team all looks Chinese; and the American team looks like all of them.” The United States, Friedman said, is the clearest example of a nation whose “strength comes from diversity.”

The most powerful nations in history have all followed a similar formula. In “Day of Empire,” a masterful survey of the rise and fall of empires from Ancient Rome to the contemporary United States, Yale law professor Amy Chua writes, “At any given historical moment, the most valuable human capital the world has to offer — whether in the form of intelligence, physical strength, skill, knowledge, creativity, networks, commercial innovation or technological invention — is never to be found in any one locale or within any one ethnic or religious group. To pull away from its rivals on a global scale, a society must pull into itself and motivate the world’s best and brightest, regardless of ethnicity, religion or background.”

In the contemporary world, no country has done this better than the United States. The U.S. remains more successful than any other nation in recruiting and retaining talented individuals from around the world — in sports, in entertainment, and most importantly in the scientific and technological fields that drive modern economies. But that lead has shrunk significantly over the past decade, with potentially serious implications for U.S. global leadership. The best university students, who once flocked to the United States, are finding other attractive options in the U.K., Australia and Canada — and even in long-closed Japan. Skilled workers, frustrated by the tight U.S. quotas on work visas and the long waits for permanent residency, are being lured by other countries that have overhauled their immigration laws and promise a smoother transition to a new life. And Chinese and Indians, the two largest groups of skilled migrants, have seen new job possibilities emerge in their own fast-growing economies, leading more to stay put or to come back home.

These trends have alarmed U.S. businesses, and some political leaders as well. Michael Bloomberg, the media magnate and outspoken mayor of New York, has warned that restrictive laws and a stifling immigration bureaucracy that drive away immigrant entrepreneurs and other skilled migrants are a policy of “national suicide.” He added, “I can’t think of any ways to destroy this country quite as direct and impactful as our immigration policy. We educate the best and the brightest, and then we don’t give them a green card.”