Rome first penetrated into the plain of the Po in 268, when, after the Senones had been defeated and the tribe exterminated, a Latin colony, Ariminum, was founded in their former territory (Livy, Per. XV; Vell. I, 14). For many years Ariminum remained only a defensive bastion of Roman rule, closing the gap between the Apennines and the sea, and thus denying entrance to Picenum and Umbria from the north. It was later that the role of Ariminum was changed into that of a springboard for the offensives that subdued the Po country. This development was at once made possible and probable, however, by the location of the colony and the orientation of its territorium.
The original size of the colony is open to dispute. What is more important for our purposes is to attempt to determine its extent to the north and the west; to know how far it stretched into the Aemilian plain. On the south the boundary was the Crustumius (CIL XI, p. 77; Pliny, HN III, 115). On the north, the boundary with Ravenna and then with Caesena was certainly the Rubicon in the late Republic and afterwards. At the time of foundation, however, there was probably a greater area under the administration of Ariminum, but perhaps not settled by colonists.