Rome had begun annexing provinces in the 3rd century BC, four centuries before reaching its greatest territorial extent, and in that sense was an "empire" while still governed as a republic.[8] Republican provinces were administered by former consuls and praetors, who had been elected to one-year terms and held imperium, "right of command".[9] The amassing of disproportionate wealth and military power by a few men through their provincial commands was a major factor in the transition from republic to imperial autocracy.[10] Later, the position of power held by the emperor was expressed as imperium.[11] The Latin word is the origin of English "empire," a meaning it began to acquire only later in Rome's history.[12]
The Augustus of Prima Porta (early 1st century AD)
As the first emperor, Augustus took the official position that he had saved the Republic, and carefully framed his powers within republican constitutional principles. He rejected titles that Romans associated with monarchy, and instead referred to himself as the princeps, "leading citizen". Consuls continued to be elected, tribunes of the people continued to put forth legislation, and senators still debated in the curia. It was Augustus, however, who established the precedent that the emperor controlled the final decisions, backed up by military force.
The reign of Augustus, lasting more than 40 years, was portrayed in Augustan literature and art as a new "Golden Age." Augustus laid out an enduring ideological foundation for the three centuries of the Empire known as the Principate (27 BC–284 AD), the first 200 years of which is traditionally regarded as the Pax Romana. During this period, the cohesion of the Empire was furthered by participation in civic life, economic ties, and shared cultural, legal and religious norms. Uprisings in the provinces were infrequent, but put down "mercilessly and swiftly" when they occurred,[13] as in Britain and Gaul. The sixty years of Jewish–Roman wars in the first half of the 2nd century were exceptional in their duration and violence.[14]
The success of Augustus in establishing principles of dynastic succession was limited by his outliving a number of talented potential heirs: the Julio-Claudian dynasty lasted for four more emperors—Tiberius, Caligula, Claudius, and Nero—before it yielded in 69 AD to the strife-torn Year of Four Emperors, from which Vespasian emerged as victor.
Vespasian became the founder of the brief Flavian dynasty, to be followed by the Nerva–Antonine dynasty which produced the "Five Good Emperors": Nerva, Trajan, Hadrian, Antoninus Pius and the philosophically inclined Marcus Aurelius. In the view of the Greek historian Dio Cassius, a contemporary observer, the accession of the emperor Commodus in 180 AD marked the descent "from a kingdom of gold to one of rust and iron"[15]—a famous comment which has led some historians, notably Edward Gibbon, to take Commodus' reign as the beginning of the decline of the Roman Empire.
In 212, during the reign of Caracalla, Roman citizenship was granted to all freeborn inhabitants of the Empire. But despite this gesture of universality, the Severan dynasty was tumultuous—an emperor's reign was ended routinely by his murder or execution—and following its collapse, the Roman Empire was
