The guard acted a kingmakers for the first time when they found Claudius hiding behind a curtain after the assassination of Caligula, and proclaimed him Caesar. I mentioned in previous posts the guard’s sinister role in the year of four emperors and the auctioning of the Empire. Gibbon comments specifically on the danger of a private army:
Such formidable servants are always necessary, but often fatal to the throne of despotism. By thus introducing the Praetorian guards as it were into the palace and the senate, the emperors taught them to perceive their own strength, and the weakness of the civil government; to view the vices of their masters with familiar contempt, and to lay aside that reverential awe,