 In the decades after the death of Augustus , the Roman Empire was , in a sense , a union of inchoate principalities , which could have disintegrated at any time . In 68 AD , 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 . 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 69 , Galba was assassinated and Otho took an army to Germany to defeat Vitellius , but instead was himself defeated . He committed suicide , and Vitellius was proclaimed Emperor , but was quickly defeated and the executed by Vespasian , who was then declared Emperor . Under the Emperor Vespasian , the Roman constitution began a slide toward outright monarchy . Vespasian died in 79 , and was succeeded by his son , Titus , who presided over a further weakening of the senate . He was succeeded by his brother , Domitian , in 81 . Domitian 's reign marked a significant turning point on the road to monarchy , as he made himself Censor for life , and unlike his father , used these powers to further subjugate the Senate . Domitian , ultimately , was a tyrant with the character which always makes tyranny repulsive , and this derived in part from his own paranoia , which itself was a consequence of the fact that he had no son , and thus no obvious heir . In September 96 , Domitian was assassinated . 
