The Roman Empire began around 27 BC, replacing the Roman Republic. During the five centuries of its existence, the empire would expand significantly. While expansion is success, it also creates a need for military strength — and that need is continuously changing.
Historian Edward Luttwak describes the geopolitical history of the empire in three phases, in order: the growth, maintenance, and defense of the empire:
The first expansive, hegemonic, and reliant chiefly on diplomatic coercion; the second meant to provide security even in the most exposed border areas, in part by means of fortified lines whose remains are still visible from Britain to Mesopotamia; and the third a defense-in-depth of layered frontier, regional, and central reserve forces that kept the western empire going till the fifth century and the eastern empire for much longer.
For roughly the first century of its existence, the military policy of the empire was expansion. The empire was birthed in the midst of the collapsing republic. One of several reasons for the end of the republic was that it was a form of government suited to ruling the city of Rome and the surrounding territories in central Italy, but it was not suited to ruling a world empire. Although the empire would ultimately rule more territory than the republic, the republic, during its last years, had control of millions of square miles, including the areas of present-day Spain, France, Greece, Turkey, and North Africa.
Maintaining effective military control of occupied regions, and mounting campaigns to gain control of still more land, was a task which required quick and consistent decision-making and a clear chain of command. The Senate, which was the main governing body during the republic, was a deliberative institution which engaged in debate. Military commanders in the field, sending requests for instructions to Rome, couldn’t wait for the Senate’s thorough discussions.
“The first system of imperial security was essentially that of the late republic, though it continued” until around 68 AD, when Nero died. This first system operated “under that peculiar form of autocracy we know as the principate.” The principate was a legal fiction created by Octavian, designed to retain the appearance of a republic while concealing the fact that the actual machinery of government was functioning as a dictatorship.
The empire was born in war, and kept a martial flavor for most of its existence, as Edward Luttwak explains:
Created by the party of Octavian, himself a master of constitutional ambiguity, the principate was republican in form but autocratic in content. The magistracies were filled as before to supervise public life, and the Senate sat as before, seemingly in charge of city and empire. But real control was now in the hands of the family and personal associates of Octavian, a kinsman and heir of Julius Caesar and the ultimate victor of the civil war that had begun with Caesar’s murder.
That war had been an internal power struggle between the three members of the Second Triumvirate: Octavian, Mark Antony, and Lepidus. In 36 BC, Lepidus was defeated by Octavian and sent into exile, where he lived safely and died of natural causes around 13 BC or 12 BC. That left Octavian and Mark Antony to face each other. Mark Antony had Cleopatra as an ally, but the war ended in 31 BC “with the final defeat of Antony and Cleopatra.”
Octavian now had all the power which was originally divided among three men: the separation of powers was destroyed, the Senate largely powerless, and Octavian, becoming emperor, chose a new name: Augustus.
Octavian-Augustus was succeeded by Tiberius, then Caligula, and later Claudius. At the death of Claudius, Nero became emperor.
Nero’s tyranny and excesses provoked rebellion. In part because Nero failed to clearly identify a successor, and in part because any successor named by him would likely not have been acknowledged, there was an open question about who would be the next emperor.
“When Nero died,” writes Luttwak, it was under mysterious circumstances. He had been obliged to flee the city of Rome because of uprisings against his tyrannical rule. Some historians write that he committed suicide; other write that the governor of a Greek island discovered him living in disguise and ordered that he be executed. Most of the evidence points toward suicide. In any case, when Nero died in 68 AD, “another had already claimed his place.”
On June 8, 68 AD, the Senate, seeing that Nero was unlikely to retain power in the face of the rebellion, named Galba to be emperor. Most evidence points toward Nero dying by suicide on June 9 in Rome. Galba, however, had been in Spain when the Senate named him emperor, and was not yet in Rome.
But the new emperor, Galba, did not arrive in Rome until October and did not live beyond January 69. M. Salvius Otho, an ex-governor of Lusitania, though in Rome as Galba’s follower, procured his murder at the hands of the Praetorians and was acclaimed emperor in turn. By then yet another had risen to claim the office, Aldus Vitellius, governor of Lower Germany and master of its four legions. So far, contention had been resolved through suicide and murder; now there was to be civil war also.
The lower levels of the Roman bureaucracy trundled along, keeping the empire running, while at the highest levels, open warfare among would-be emperors manifested itself in a string of assassinations. After Nero’s death, Rome would have four different emperors within a year.
The military, especially those units on the frontiers, had no steady source of instructions. It engaged largely in holding actions during this time. The era of grand expansion was over.
The outstanding virtue of the principate, the constitutional device invented by Augustus, was its reconciliation of republican forms and traditions with autocratic efficiency. Its outstanding defect was that the succession was dynastic, but without any mechanism to secure it as such, or to replace an unfit dynast. When a tolerable emperor chose a capable successor and made him a son by adoption, all was well. Adoption satisfied the dynastic sentiments of the army and the common people without offending the anti-dynastic prejudice of the Senate. But if there were no adequate son and none were adopted, he became emperor who could make himself emperor; usually by force.
During the next two centuries, succession was not always as chaotic and violent as it was after the death of Nero. But the flaws in Augustus’s precedent had been visible, and succession remained a point of contention for the remainder of the empire.
Although there would be occasional bursts of expansion, the empire energy and focus could not be as devoted to conquest as it had been earlier, because some amount of attention was continuously absorbed by the matter of succession.
