The Donativum
The Roman Legion and particularly the Generals of the Roman Legion became very popular in Rome and they were in close consort with the Emperor. The Praetorian Guard, the bodyguards of the Emperor were also rising in power during the 2nd and 3rd centuries. They even became part of the process for appointing new emperors. Therefore it became imperative for any Emperor to get on the good side of the Generals and the Praetorian Guard. To do this it was decided that a sum of money or lavish gifts would be given to these prominent men whenever a new emperor came into power. It would be a bribe of sorts to ensure the loyalty of the army.
However it also placed substantial incentive for the Praetorian Guard and the Roman Legion to see to it that a new Emperor was put in place. The existence of the donativum led to the Praetorian Guard becoming corrupt. As the personal guards of the emperor they were in an easy position to murder him if they decided they wanted another donativum. The Praetorian Guard and some of the generals of the Roman Legion became very corrupt and even plotted the murders of several emperors in order to get the donativum. The Guard even helped to decide who would replace a slain emperor by picking the candidate that offered the largest donativum. After the death of Caligula in 41 the Guard supported Claudius and the Senate eventually learned that the Guard had installed Claudius on the throne. His promised donativum was one of the largest in recorded history. By the late history of the Western Roman empire the succession of the emperor had become very chaotic and the Emperor would often be more in fear of his guard than feel protected by them.