What to make, then, of President Obama's pep talk last month to U.S. troops in Afghanistan in which he lauded them as "the finest fighting force that the world has ever known"? ...

In fact, this sort of description of the U.S. military has become something of a must for American presidents. Obama's predecessor George W. Bush, for example, boasted of that military as alternately "the greatest force for freedom in the history of the world" and "the greatest force for human liberation the world has ever known." Hyperbolic and self-promoting statements, to be sure, but undoubtedly sincere, reflecting as they do an American sense of exceptionalism that sits poorly with the increasingly interconnected world of the twenty-first century.

... Still, the gold medal for the largest land empire in history -- and the finest fighting force of all time -- must surely go to the thirteenth century Mongols.

Led by Genghis Khan and his successors, Mongol horsemen conquered China and the Islamic world -- the two most powerful, sophisticated civilizations of their day -- while also exerting control over Russia for two and a half centuries. And thanks to a combination of military excellence, clever stratagem, fleetness of foot (and far more important, hoof), flexibility, and when necessary utter ferocity, they did all this while generally being outnumbered by their enemies.