It was a municipium with an extensive territory, and of some importance under the Empire, but was plundered by the partisans of Otho in A.D.
Before the Roman conquest it was probably a native capital: afterwards it received the dignity of a municipium (implying municipal status and Roman citizenship).
It must have become a municipium by the lex Julia of 90 B.e., and it was here that Julius Caesar in 56 B.C. held his famous conference with Pompey and Crassus, Luca then being still in Liguria, not in Etruria.
By the Lex Julia of 90 B.C. and the Lex Plautia Papiria of 89 B.C. every town in Italy which made application in due form received the complete citizenship. The term municipium was no longer confined to a particular class of Italian towns but was adopted as a convenient name for all urban communities of Roman citizens in Italy.
It was already a municipium in the time of Augustus, and enjoyed great prosperity under later emperors.
It was a municipium under the early empire, but was converted into a colonia under Antoninus Pius by Herodes Atticus, who provided it with a water-supply.
All we know of their subsequent political condition is that after the Social war the folk of Cliternia and Nersae appear united in a res publica Aequiculorum, which was a municipium of the ordinary type (C.I.L.
The inscription by which the existence of a Jassiorum municipium in the time of the Roman Empire is sought to be proved, lies open to grave suspicion; but the city is merrtioned as early as the 14th century, and probably does derive its name from the Jassians, or Jazygians, who accompanied the Cumanian invaders.
It was originally a Latin colony, but became a municipium probably in 90 B.C. The customs boundary of Italy was close by in Cicero's day.
Taramelli (Notizie degli Scavi, 1905, 41 seq.) rightly points out that the nucleus of the Roman municipium is probably represented by the present quarter of the Marina, in which the streets intersect at right angles and Roman remains are frequently found in the subsoil.