'The Roman Empire' may refer either to the extensive overseas territories which Rome ruled for much of its history or to a period in Roman history, namely, the centuries after 27 BCE when Rome no longer had a republican form of government but was ruled by emperors.
- Rome's overseas territories
Rome began to acquire an empire in the third century BCE as a result of its victories over the North African city of Carthage in the First and Second Punic Wars (264-241 and 218-201 respectively). Its first acquisitions were the islands of Sicily (241 BCE), Sardinia (231 BCE), and Corsica (231 BCE), and a large part of the Iberian peninsula (197 BCE). In the second century BCE Rome conquered and annexed territories which roughly correspond to modern Greece, Albania, and part of the former Jugoslavia, as well as the western half of Asia Minor, and a small part of North Africa (roughly modern Tunisia), and so by 100 BCE the Roman Empire extended through the lands bordering the Mediterranean Sea from the eastern half of the Iberian peninsula to the western half of modern Turkey. Just over a century later, at the death of the first emperor Augustus in 14 CE, the Empire had expanded to include the whole of the Iberian peninsula in the west, the whole of Asia Minor as well as Syria and Palestine in the east, the territories corresponding to modern Holland, southern Germany, Austria, Hungary, and Romania in the north, and the eastern half of North Africa (i.e., the territories corresponding to modern Egypt, Libya, and Tunisia) in the south. The Roman Empire reached its furthest extent under the emperor Trajan (reigned 98-117), though most of his conquests were not retained within the empire for long. Even so, by the middle of the second century CE Britain (England and southern Scotland), mainland Europe south of the rivers Rhine and Danube, southwest Asia, and Africa north of the Sahara were subject to the rule of Rome, and remained so for the next three centuries. Towards the end of the third century the emperor Diocletian (reigned 284-305), recognising the difficulty of administering so vast an extent of territory, divided the Roman Empire into an eastern half and a western half, each half being under the authority of two emperors - for further details see below. The last of the Roman emperors in the west, Romulus Augustulus, was deposed by the German general Odoacer in 476, and this event is conventionally taken to mark the end of the Western Roman Empire. The emperor in Constantinople, however, continued to exercise authority over the eastern half of the Empire, and the Eastern Roman Empire (more commonly known as the Byzantine Empire) continued for a further thousand years until the Ottoman Turks captured Constantinople in 1453.
- The imperial period of Roman history
According to tradition the city of Rome was founded in 753 BCE and for the first two-and-a-half centuries of its history was ruled by kings, the last, Tarquinius Superbus (Tarquin the Proud), being expelled a few years before the end of the sixth century BCE. For the next five centuries Rome was a republic, with power effectively in the hands of a small circle of aristocratic families. After the assassination of Julius Caesar in 44 BCE there was a period of civil war, before in 27 BCE Caesar's great-nephew and adopted son Octavian became emperor, taking the name Augustus. From then until 476 CE Rome was ruled by emperors. Thus historians of Rome sometimes contrast (the period of) the 'empire' with (the period of) the 'republic'.
Towards the end of the third century the emperor Diocletian (reigned 284-305), recognising that the task of ruling Rome and its extensive overseas territories could not be carried out efficiently by a single emperor, introduced a new system of imperial government under which there were four emperors, two 'senior' emperors (known as Augusti), one responsible for the western half of the empire and one responsible for the eastern half, and two 'junior' emperors (known as Caesares), each to assist one of the 'senior' emperors. This new system was associated with other changes in the nature of imperial rule. In order not to antagonise unnecessarily the old aristocratic families, who resented their loss of political power, Augustus had been anxious to preserve so far as possible the forms of republican government, and for this reason wished to be regarded, and referred to, as princeps (literally 'first' or 'first citizen'), preserving the illusion that he was not an autocratic ruler but merely the first citizen within a republic. By the time of Diocletian, however, the emperors no longer felt the need to maintain this fiction, and from Diocletian's time onward the emperors were openly acknowledged as absolute rulers. Historians of Rome sometimes mark this distinction between the two periods of imperial rule by referring to the period before Diocletian as the principate (i.e., the period when the emperor was still regarded (merely) as princeps). (The Latin word princeps is the origin of the English word 'prince'.)