From Tiberius To Vitellius
When Augustus died in 14 AD, the Principate legally ended. While Augustus had granted Tiberius the legal standing that he needed in order to become Princeps (i.e., Roman Emperor), Augustus could not legally make Tiberius Princeps. However, Tiberius' legal powers, as well as his status as the chosen heir of Augustus, gave him an opportunity that his ambition could not refuse. Tiberius knew that if he secured the support of the army, the rest of the government would soon follow. Therefore, Tiberius assumed command of the Praetorian Guard, and used his Proconsular imperium to force the armies to swear allegiance to him. As soon as this occurred, the senate and the magistrates acquiesced. Tiberius' efforts were so successful, that when the senate declared him Princeps, he made his acceptance appear to be a concession to the demands of the senators. Under Tiberius, the power to elect magistrates was transferred from the assemblies to the senate. Now, the assemblies were only used to hear the results of magisterial elections. In addition, they did retain some theoretical legislative powers. When Tiberius died, Caligula was proclaimed emperor by the senate. Caligula transferred the electoral powers back to the assemblies, but then quickly returned those powers to the senate. In 41 Caligula was assassinated, and for two days following his assassination, the senate debated the merits of restoring the republic. Due to the demands of the army, however, Claudius was ultimately declared emperor. Claudius' antiquarian interests resulted in his attempts to revive the old Censorship, and to return some degree of independence back to the senate. Ultimately, Claudius was killed, and Nero was declared emperor.
Arguably, the most significant flaw in the constitution left by Augustus concerned the matter of succession. This deadly flaw was violently exposed in the year 69. Augustus had established a standing army, where individual soldiers served under the same military governors in the same provinces over an extended period of time. The consequence was that the soldiers in the provinces developed a degree of loyalty to their commanders, which they did not have for the emperor. Thus the empire was, in a sense, a union of inchoate principalities, which could have disintegrated at any time. The first indication of a nationalist movement appeared in Gaul (modern France) in 68, but this movement ended when its leader, C. Julius Vindex, was defeated by an army under L. Verginius Rufus. Rufus was the governor of Upper Germany, and while he was declared imperator by his soldiers, he decided not to use his support to march on Rome and make himself emperor. He did not decline this opportunity because he was loyal to the emperor Nero, but rather because of his own low birth, and his belief that his low birth might make it difficult for him to accomplish anything as emperor. Shortly after Rufus had been declared imperator, Ser. Sulpicius Galba, the governor of Hispania Tarraconensis, was proclaimed emperor by his troops. In Rome, the emperor Nero quickly lost his supporters and committed suicide. Galba, however, did not prove to be a wise leader. He chose to punish Rufus' troops, and to antagonize the Praetorian Guard by not fulfilling promises which had been made to them.
The governor of Lower Germany, A. Vitellius, was soon proclaimed emperor by his troops, and in Rome, the Praetorian Guard proclaimed M. Salvius Otho emperor. In January 69, Galba was assassinated, and the senate proclaimed Otho emperor. Otho took an army to Germany to defeat Vitellius, but was himself defeated by Vitellius. He committed suicide, and Vitellius was proclaimed emperor by the senate. Another general, Vespasian, soon defeated Vitellius. Vitellius was executed, and Vespasian was named Augustus, elected Consul, and given Tribunician powers. The result of these events illustrated a key weakness in Augustus' constitution. The method of succession was in part based on heredity, and if a dead emperor's next-of-kin had the support of the Praetorian Guard, then his installation as emperor was reasonably assured. However, with Nero's death, the Julian line had been extinguished. Thus, the events of 69 showed that the armies in the provinces were the ultimate 'electoral bodies', and that any successful general could legitimately claim a right to the throne. It also became apparent that, while the senate might acquiesce to the installation of a new emperor, such acquiescence was simply a matter of form. This flaw would contribute to the ultimate fall of the Roman Empire.
Read more about this topic: History Of The Constitution Of The Roman Empire