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As the United States of America celebrates its 236th birthday today, a recurring question ringing across this vast country, and indeed across the world, is whether the superpower has lost its mojo, an Afro-Caribbean word that originally meant charm or spell, but now encompasses creative genius, spark, or even hunger for success. In politico-economic talk shows and geostrategic gabfests, the inevitability of American decline is a persistent theme, to the extent that President Obama, accused by critics and opponents of easing America's descent into the commonplace, has had to contest the charge repeatedly and warn against writing off the country. He's not alone. Warren Buffett heard the talk of American decline three years back and cautioned: It's never paid to bet against...