The Praetorian guard were the personal bodyguard of the emperors of Rome. While no military forces were allowed south of the Rubicon river, in other words in Italy itself, the guard was a way to get around this. Starting with the very first emperor Augustus, the size of the guard grew from 5000 to 10000 men.
While they had no direct say in the government, they held great sway at different times in the history of the empire due to being the only military body near the emperor. It was through a coup of a portion of the guard that the third emperor of Rome, Caligula was assassinated. They then showed their power by declaring Claudius as the fourth emperor of Rome, a proclamation that proved to become reality as he reigned for 13 years. They deserted Nero to the Roman mobs and during the anarchy that followed, first supported Galba for emperor, then turned on him and supported Otho. They were not above bribes and supporting the highest bidder. It was expected an increase in pay or a bonus would be given for any candidate they successfully supported for Emperor.
Over the years their power and position varied. Some emperors disbanded them and replaced them with totally different men. The last Emperor to have a Praetorian guard was Constantine, who disbanded the group in the early 300's when changing political realities made them unnecessary.
- Dupuy & Dupuy, The Encyclopedia of Military History, 1977
- William L. Langer, An Encylopedia of World History, 1948
- Will Durant, Caesar and Christ, 1944