Geographically, the Ager Romanus (Italian - Agro Romano) is the name given to the immense rural area (part plains, part hilly) around the city of Rome. Politically and historically, it has represented the area of influence of Rome's municipal government. It is delimited to the south by Monti Prenestini, by the Alban hills and the pianura pontina; to the west by the Tyrrhenian Sea; to the north by the hills surrounding Lake Bracciano and to the east by the Monti Tiburtini.
History
Ancient Rome
The Rome of Romulus at his immediate successors possessed a very restricted territory, as did the other neighbouring Latin cities such as Praeneste: and this territory was delimited by boundary stones, or cippi, delimiting the area of influence of a city or a private citizen.
The legend holds that a question of confines (and of expanding beyond them) led to Tullus Hostilius declaring war on Alba Longa, destroying this Latin city and incorporating its territory in that of Rome, around 635 BC.
With the proclamation of the Roman Republic in 510 BC, all the territory occupied by Romans in "Latium vetus" came to be proclaimed ager publicus, equivalent to state lands today, which were held by the state and could be granted to private citizens. The Roman municipal authorities of this era were the consuls. In effect, Rome was a gigantic city-territory continuously expanding across Europe.
Octavian Augustus founded the office of praefectus urbis and other offices which divided the administration of the city of Rome from that of the Roman Empire. Thus was solved the problem of delimiting the territory of the municipium of Rome from the territory of the rest of the empire - besides the Regio I Latii et Campaniae administered by a specific governor, the confines of the municipal authority of Rome came to be fixed at the "centesimum lapidem" (i.e. one hundred miles) on each of the via consularis converging on Rome. So, de jure, the Roman municipal authority controlled the whole of Lazio and part of Tuscany from Talamone to Terracina and also parts of Abruzzo and Umbria.
The same territorial division was confirmed by the re-subdivision of the provinces by Diocletian.
Medieval era
After the fall of the empire, the praefectus urbis continued to be elected and did not come to be totally deprived of their power even by the advancing papal power which effectively became the municipal government of Rome.
The Ager Romanus, as a political zone subject to the municipium of Rome, theoretically continued to extend ad centesimum lapidem, but in practice many of its regions ended up in Lombard hands, with still others managed by the religious authorities or the pope, who were beginning to manage their territories by means of patrimonia and domuscultae.
Today
The motu proprio of 6 July 1817 by Pope Pius VII established the boundaries of the municipium of Rome, assigning to the capital its present communal territory as well as the present communes of Fiumicino, Pomezia and Ardea. The confines of the Comunità di Roma were thus finally defined and no longer nebulous, and these limits ended at the constitution of the commune of Pomezia (also including present-day Ardea) following its lasting foundation during the "bonifica fascista" and - in the nineties - of Fiumicino.
Source
- This page is a translation of its Italian equivalent.