COMMENTS on the visible decline and, in the most optimistic estimates, impending demise of the American empire often refer to the Roman Empire 2,000 years ago as a cautionary precedence. And the notorious emperors Nero and Caligula are occasionally compared with the incumbent US president. Those comparisons might not be ridiculous, but there is potentially a closer, albeit lesser known, analogue. Commodus became co-emperor in his early teens during the reign of his revered father, Marcus Aure...