The Praetorian Guard killed four emperors, elevated at least five more to power, and constituted the single most politically destabilizing institution in Roman imperial history. This was not a design intention. Augustus established the Guard as a personal security force — a professional bodyguard organized on military lines and stationed near Rome — because the emperor needed reliable protection and the Republic’s tradition of civilian governance had made no provision for one. What Augustus created as a security measure, his successors inherited as a power center whose loyalty could be purchased, whose commanders accumulated enormous influence, and whose physical proximity to the emperor gave it an influence over succession that no amount of constitutional theorizing could override.

The Roman Empire never solved its succession problem. This was not an oversight or a failure of political imagination — it was a structural consequence of the way the principate was constructed. Augustus had built a system that was functionally monarchical but constitutionally republican, which meant it could not have formal hereditary succession without admitting it was a monarchy. The result was a fiction maintained for centuries: that each emperor received his powers from the Senate and people of Rome, and that the previous emperor’s designation of a successor was a recommendation rather than a binding determination. Everyone knew this was a fiction. The fiction was maintained because the alternative — acknowledging that Rome was a hereditary monarchy — was politically untenable for an aristocratic culture that had executed men for aspiring to kingship.