The surest way to shape one out of all the manifold peoples of the world – as Posidonius had long before pointed out to Pompey – was for Rome to rule the lot. In 212, an edict was issued that would have warmed the old Stoic’s heart. By its terms, all free men across the vast expanse of the empire were granted Roman citizenship. Its author, a thuggish Caesar by the name of Marcus Aurelius Antoninus, was a living embodiment of the increasingly cosmopolitan character of the Roman world. The son of an African nobleman, he had been proclaimed emperor in Britain, and was nicknamed Caracalla –
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