 When Augustus died in 14 AD , the Principate legally ended . 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 . Under Tiberius , the power to elect magistrates was transferred from the assemblies to the Senate . When Tiberius died , Caligula was proclaimed Emperor by 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 declared emperor , but he was ultimately killed , and Nero was declared Emperor . 
