During most of the Republic there had been no government as moderns would understand the term. There were no fully staffed departments or agencies whose existence continued independently of the political situation. Instead the Republican ideal had been to elect certain magistrates with various powers to carry out the token duties of the city-state government. The leading posts (Consuls and Praetors) combined military and civil responsibilities. If elections could not occur for whatever reason to bring these individuals to the helm, there was no government in any meaningful sense of the word.
Augustus Caesar reorganized the government. While outwardly adhering to traditional republican principles, there were nonetheless the first stirrings of government in a manner that could be recognized by moderns. Among Augustus Caesar's innovations was a city prefect, a postal system, and something resembling a fire brigade (vigiles) Furthermore, around the figure of the "Princeps" or first-citizen, who ruled more from his spirituality authority (auctoritas) rather than through naked displays of power, there emerged something resembling an imperial court. These central officials began to assume the increasingly complex legal and political administration of an empire that now spanned three continents. Incorporating Senators, Equestrians, freedman and even educated slaves, a nascent civil service emerged. The now professional legions were quartered on the frontiers, and ruled by the Princeps through various legates.
Three centuries later the empire had slowly evolved into an organism which began to outstrip Augustus Caesar's visions. The various civil wars and rebellions had demonstrated the limits of the Princep's spiritual authority to impose order. In any event, the rise of a regional superpower in the form of Sassanid Persia obligated a drastic change in the nature of civilian and military command. The Crisis of the Third Century nearly tore Rome apart. From Aurelian's desperate efforts to save the Empire, Diocletion accelerated, and Constantine finalized, various trends that would reshape Imperial society.
Broadly, these trends were:
1) Replacing the nebulous auctoritas of the central ruler with the quasi-divine stature of a Hellenistic monarch
2) centralization of political authority within the respective halves of the empire
3) A growing and structured division between civil and military commands
4) The political decline of a privileged landowning class in favor of an expanding military and legal bureaucracy
Did you know...?
The praefectus aegypti, was considered the highest ranking equestrian post during the early empire. Later, the post would fall second to that of the praetorian command, but its position remained highly prestigious.