Flaw. One weakness of the "principate"—Augustus's disguised monarchy—was that it did not deal well with the problem of succession. Officially, there was no emperor, just a man who happened to hold an extraordinary set of powers and offices. As a result, there could be no official rules for succession (contrast, for example, the elaborate scheme that can tell who is twelfth in line for the British throne today). Many emperors were able to choose a successor, usually a son, and have him granted a similar package of powers in advance. But what if there were no designated successor? Several emperors and would-be emperors were acclaimed by the Praetorian Guard (the imperial bodyguards) or various armies in the field. The candidates could be members of the existing imperial family or simply men who happened...