Diocletian accelerated a notable trend in imperial politics, which was to split provinces into ever smaller units, each with its own governor and staff. This was meant to ensure a greater efficiency in collecting tax revenues, as financial power now lay with the provinces rather than the municipalities . By Diocletian's time there was now some hundred provinces. The provinces were grouped into twelve intermediary administrative units called dioceses.
The tax unit was now based on the iugum, which assessed the value of land according to its productivity rather than solely from its area. Also added was the caput, the value of "human resources". Any changes in the tax rates were now the responsibility of the Praetorian Prefects.
Diocletian more or less accepted the borders of the empire. He inaugurated a massive project of defensive fortifications along the Germanic and Persian frontiers. These garrison troops (limitanei) were now permanent fixtures of the Empire. They seemed adequate enough to deal with Germanic raiding parties or Persian assaults. Should something break through the border defenses, there was now a mobile field army (comitatenses) attached to strategic Imperial centers. This solved the main weakness of Augustus' defensive system - no central reserve of troops. Furthermore, for reasons political and well as martial, command of provincial military affairs was now usually separate from the civilian administration.
Critics have contended the expanded government and military ate more taxes than it produced, and others alleged an increase in government corruption. However, the central functions of the reform were to ensure the military and financial solvency of the empire to deal with post-Crisis threats. In this the new system seems to have faired adequately. There is no real evidence the system would have collapsed were it not for the unforeseen catastrophe of Germanic supertribes created indirectly by the Huns.
Augustus had taken Caesar's murder to heart and not granted himself anything approaching titles of royalty. Instead various republican powers had been concentrated into one persona. This persona was often called Princeps or leading citizen, as well as Imperator or victorious general.
Gradually the mask wore off. Diocletian was the first to call himself Dominus, or Lord over all. Thus the later empire is often termed the Dominate. In addition to the more commanding title, the pomp and ceremony of Hellenistic god-kings were grafted to the imperial court. Debasement and servility were owed to the Dominus, who was regarded now as a quasi-divine ruler.
This self-aggrandizement is understand best as an attempted remedy to the limits of the more abstract authority wielded by the Princeps. But reality would demonstrate the new visage of the Dominus was not enough to entirely forestall rebellion and usurpers. A weak Dominus could find himself the figurehead for a generalisimo - or even for influential females in the imperial family. The Hellenistic East, however, would eventually take the logic to its ultimate conclusion and proclaim their ruler Basileus, or king.
Diocletian presided over a novel experiment in which the two halves of the empire were each ruled over by a senior ruler, both of whom also retained a junior emperor who acted as heir and lieutenant. While this four man rule ultimately proved unwieldy, after Diocletian there were usually two leaders, each stationed in one half of the empire. The threat of Persia in particular necessitated a separate command for the East. Thus while the empire finally split politically along its Latin-Hellenic cultural axis, within those respective spheres the new figure of the Dominus facilitated centripetal trends.
The Kings of Rome, as well as the early Consuls, had chosen leading citizens of the city-state to form an advisory body. Gradually, the Senate became a body of ex-magistrates whose collective "advice" proved quite influential on the current magistrates. The Senate was the only deliberative body in the Republic, and the only one with the expertise to deal with the more complicated matters of state. The Senate was largely composed of Rome's socio-political landowning elite, with the same families having produced senior magistrates for generations (though occasionally a "new man" would come to the fore). During the various travails of the Late Republic, the Senate often cowered before powerful warlords. It was not so much the Senate's "advice" that was sought, but the dignity of the body which could be used to confer legitimacy on a military strongman.
The Augustan reorganization found a new vision for the Senate. The Senate became a corporate social body for the Empire's economic elite, with entry qualifications set as owning 1 million sesterces. Senators were allotted many privileges in imperial society, but were also subject to the August's stern moralist legislation. Augustus allowed the Senate only minor powers, but from its ranks the Princeps would draw certain trusted individuals to staff military and civil commands. With the civil wars having exterminated much of Rome's ancient aristocracy, local elites were gradually incorporated into the body, transforming the Senate into an imperial institution. The Senate's power lay still in its dignity as a corporate body, and its ability to confer legitimacy to the reigning Imperator. It was ranking senators who took to the battlefields to decide questions of imperial succession.
After the crisis of the Third Century, the Senate quickly lost its even nominal powers. The Senatorial order was increasingly blurred with the lesser Equestrian order (see below). The Dominus began transferring most civil and military commands to his trusted military companions, to whom he offered the title of "Senator" as a pure honorific. The Imperial court was no longer even in Rome, and the Roman Senate founds itself an anachronistic body. Constantine upgraded Constantinople's city council to the status of a Senate, but it too held little more than ceremonial powers in his new regime. The aristocratic body of landowners that had advised Roman civilization for so long was replaced by a band of rough-and-tumble military functionaries.
During the early days of Rome, those who had been rich enough to own a horse were designated into the military classes as "knights". As Rome relied more on its allies for cavalry, the knights lost military importance. The knights were nonetheless still wealthy and socially prominent, and the title "equestrian" served as a status of nobility.
While initially synonymous with the Senatorial party, as Rome expanded there developed a fine distinction. The highest class of elite held power through landowning and influence over the election of magistrates; these were called Senators. A somewhat lesser order was more apt to engage in commerce than in politics; this order became known as the Equestrians. While the two held much in common, there were points at which their interests diverged - most notably tax farming in the Asian provinces.
Augustus redefined the Equestrian order as those in imperial society owning 400,000 sesterces. Equestrians constituted the second class of imperial society behind Senators, and included many Italians and provincials. While many Equestrians still engaged in commerce, others were employed by Augustus to staff lower level civil and military commands. In some instances where Senators could not be trusted to command a particularly sensitive post, an Equestrian was appointed instead. These few equestrians serving in key posts held more actual power than most Senators.
A common military man could be raised to the level of Equestrian if he managed to attain the rank of senior centurion (Primus Pilus). This became increasingly common, and after the Crisis of Third Century, military men who had attained equestrian rank in this way replaced Senators as army commanders and provincial governors. Diocletian's administrative staff was composed primarily of these military equestrians. At the same time, it became common practice to elevate the holder of a high office into the Senatorial party if he had not already been a Senator before holding the post. These two trends merged, and there was no longer a clear distinction between the Senatorial and Equestrian orders. After Constantine, the distinction between Senator and Equestrian had been largely replaced by the three grades of Count (comes), or companions of the Emperor.
Under the Principate, decoriones were town councilors, the third level of the imperial ruling class behind Senators and Equestrians. The local elites contributed their own private funds to the state; in return, they were granted administrative powers at the municipal level. Local elites serving as decuriones were a major component of Romanization in the early empire. As decuriones manipulated imperial finances at the local level, they were in a position to accept and confer favors to clients; thus the position was highly sought after. At first decuriones were elected by local popular assemblies, but the position gradually became hereditary.
After the Crisis of the Third Century, imperial finances were reorganized. In an effort to maintain the expanded military necessary to counter the Persian threat, finances were handled less at the local level and more at the provincial. This centralization robbed the position of decurion of most of its prestige. Consequently, local elites increasingly sought to serve in the provincial government or the imperial bureaucracy. Diocletian and his successors found it necessary to use persuasion to retain some of the local elites as town councilors. A position that had once been honored under the Principate came to be seen increasingly as second-rate.
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Officially, the Equestrians were the second tier of the elite. In the theaters and amphitheaters, they occupied ranks behind the Senators.