After the Praetorian Guard was established under Augustus , Sejanus introduced reforms which saw the unit go from a mere bodyguard to an influential branch of the government . It controlled public security , and influenced civil administration . While the Praetorian Guard was formally established under Emperor Augustus , Sejanus introduced a number of reforms which saw the unit evolve beyond a mere bodyguard into a powerful and influential branch of the government involved in public security , civil administration , and ultimately political intercession ; changes which would have a lasting impact on the course of the Principate .