Former President Barack Obama liked to quote the Rev. Martin Luther King Jr.'s line that "the arc of the moral universe is long, but it bends toward justice." Of course, what was an expression of optimism for the moral reformer King was more like a victory spike of the football for a competitor in a zero-sum electoral contest like Obama.

More importantly, the belief that things are moving toward justice comes more naturally to believers in American exceptionalism, of which Obama isn't one. He famously said that he believed in America's exceptionalism only as much as "Brits" believe in British exceptionalism or Greeks believe in Greek exceptionalism.

But British and Greek exceptionalism look to distant pasts and encompass the idea of decline, which, if not the opposite of justice, is certainly not positive. Britons may look with pride on the British Empire, but not without a twinge of regret that it has all but disappeared. Greeks may look back on the astonishing creativity of Athens 2,500 years ago, but not without recognizing that it was held down under the Roman, Byzantine and Ottoman empires for almost all the centuries since.

In contrast, the United States has a history that can be easily, if perhaps oversimply, interpreted as a story of continual winning. Economic growth — the exception rather than the rule before 1800 in European lands — has been cumulative over time. Cultural progress abounds: the abolition of........

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