Italy (Italia in Latin and Italian) was the name of the administrative division of the Italian peninsula during the Roman era. It was not a province, but became the territory of the city of Rome, thus having a special status. Following the end of the Social War (91–88 BC), Rome had allowed its Italian allies (socii) full rights in Roman society and granted the Roman citizenship to all the Italic peoples. After having been for centuries the heart of the Empire, from the 3rd century the government and the cultural center began to move eastward: first the Edict of Caracalla in 212 extended Roman citizenship to all free men within the imperial boundaries, then Christianity became the dominant religion during Constantine's reign (306–337) raising the power of other Eastern political centres, and finally the capital was moved to Constantinople in 330. As a result, Italy began to decline in favour of the provinces, which resulted in the division of the Empire into two separated entities in 395: the Western Empire, with capital Mediolanum (Milan), and the Eastern Empire, with capital Byzantium (Constantinople).
The name Italia covered an area whose borders evolved over time. According to Strabo's Geographica, before the expansion of the Roman Republic, the name was used by Greeks to indicate the land between the strait of Messina and the line connecting the gulf of Salerno and gulf of Taranto (corresponding roughly to the current region of Calabria); later the term was extended by Romans to include the Italian peninsula up to the Rubicon, a river located between Northern and Central Italy. In 49 BC, with the Lex Roscia Julius Caesar gave Roman citizenship to the people of the Cisalpine Gaul, while in 42 BC the hitherto existing province was abolished, thus extending Italy to the north up to the southern foot of the Alps.   Under Augustus, the peoples of today's Aosta Valley and of the western and northern Alps were subjugated, and the eastern border was brought to the Raša river in Istria;  finally, in the late 3rd century AD Italy came to include the islands of Corsica et Sardinia and Sicily, as well as Raetia and part of Pannonia to the north.
At the beginning of the imperial era, Italy was a collection of territories with different political statuses. Some cities, called municipii, had some independence from Rome, while others, the coloniae, were founded by the Romans themselves. Around 7 BC, Augustus divided Italy into eleven regiones, as reported by Pliny the Elder in his Naturalis Historia:
- Regio I Latium et Campania
- Regio II Apulia et Calabria
- Regio III Lucania et Brutium
- Regio IV Samnium
- Regio V Picenum
- Regio VI Umbria et Ager Gallicus
- Regio VII Etruria
- Regio VIII Aemilia
- Regio IX Liguria
- Regio X Venetia et Histria
- Regio XI Transpadana
Italy was privileged by Augustus and his heirs, with the construction, among other public structures, of a dense network of Roman roads. The Italian economy flourished: agriculture, handicraft and industry had a sensible growth, allowing the export of goods to the other provinces. The Italian population may have grown as well: three census were ordered by Augustus, to record the number of Roman citizens throughout the empire. The surviving totals were 4,063,000 in 28 BC, 4,233,000 in 8 BC, and 4,937,000 in AD 14, but it is still debated whether these counted all citizens, all adult male citizens, or citizens sui iuris. Estimates for the population of mainland Italy, including Cisalpine Gaul, at the beginning of the 1st century range from 6,000,000 according to Karl Julius Beloch in 1886, to 14,000,000 according to Elio Lo Cascio in 2009.
In 212, when the Edict of Caracalla gave the Roman citizenship to all the people within the empire regardless of their ethnicity or social status, Italy began to decline in favour of the provinces. The following crisis of the Third Century was a period in which the Roman Empire nearly collapsed under the combined pressures of invasion, civil war, plague, and economic depression. In 284, the rise of Diocletian began a series of reforms to restoring civil order. He divided the Empire into four parts and several dioceses, the so-called Tetrarchy. Though still keeping Rome as the official capital, he chose two other residences for the Augusti: Maximian, who was responsible for the West, was installed at Milan in northern Italy, in order to prevent German invasions; Diocletian established himself at Nicomedia, in Anatolia and close to the Persian frontier, in order to keep watch on the East.
Under Constantine's reign, the Diocesis Italiae was eventually subdivided into two zones:
- Italia suburbicaria ("under the government of the urbs", i.e. Rome)
- Italia annonaria ("in charge of the provisions"), with capital Mediolanum (Milan)
Western Roman Empire
In 330, Constantine moved the capital of the empire to Constantinople, with the imperial court, economical administration, as well as the military structures (as the Roman fleets in Misenum and Ravenna). After the death of Theodosius in 395 and the subsequent division of the empire, Italy became part of the Western Roman Empire. Then came the years of the barbarian invasions, and the western capital was moved from Mediolanum to Ravenna in 402. Alaric, king of Visigoths, sacked Rome itself in 410; something that hadn't happened for eight centuries. Northern Italy was attacked by Attila's Huns, and Rome was sacked again by the Vandals under the command of Genseric in 455.
According to Notitia Dignitatum, one of the very few surviving documents of Roman government updated to 420s, Italy was governed by a praetorian prefect, Prefectus praetorio Italiae (who also governed the Diocese of Africa and the Diocese of Pannonia), one vicarius, and one comes rei militaris. The regions were governed by eight consulares (Venetiae et Histriae, Aemiliae, Liguriae, Flaminiae et Piceni annonarii, Tusciae et Umbriae, Piceni suburbicarii, Campaniae, and Siciliae), two correctores (Apuliae et Calabriae and Lucaniae et Bruttiorum) and seven praesides (Alpium Cottiarum, Rhaetia Prima and Secunda, Samnii, Valeriae, Sardiniae, and Corsicae).
In the fifth century, with the Emperors controlled by their barbarian generals, the imperial government weakly controlled Italy, whose coasts were continuously under attack. In 476, with the resignation of Romulus Augustulus, the Western Roman Empire had formally fallen. For 77 years, Italy stayed united first under Odoacer, then under the Ostrogothic Kingdom. In 554, Eastern Roman Emperor Justinian I reconquered Italy keeping in general the organization of Diocletian, but during the Lombard invasion in 568, the Byzantines were to lose most of Italy, except the territories of the Exarchate of Ravenna.
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