They served as allies of Mithradates VI Eupator, king of Pontus (in what is now western Turkey), in his wars against the Romans (c. 88-84 BC). In 78-76 BC, the Romans sent a punitive expedition over the Danube in an attempt to overawe the Jazyges.
The prime enemy of Rome along the lower Danube at this time were the Dacians. In 7 BC when the Dacian kingdom built up by Burebista began to collapse, the Romans took advantage and encouraged the Jazyges to settle in the Pannonian plain, between the Danube and the Tisza (Theiss) Rivers.
They were divided into freemen and serfs (Sarmatae Limigantes). These serfs had a different manner of life and were probably an older settled population, enslaved by nomadic masters. They rose against them in 34 AD, but were repressed by foreign aid.
The Romans wanted to finish off Dacia, but the Iazyges refused to cooperate. The Iazyges remained nomads, herding their cattle across what is now southern Romania every summer to water them along the Black Sea; a Roman conquest of Dacia would cut that route. The Roman emperor Domitian became so concerned with the Iazyges that he interrupted a campaign against Dacia to harass them and the Suebi, a Germanic tribe also dwelling along the Danube.
In early 92, the Iazyges, in alliance with the Sarmatians proper and the Germanic Quadi, crossed the Danube into the Roman province of Pannonia (mod. Croatia, northern Serbia, and western Hungary). In May, the Iazyges shattered the Roman Legio XXI Rapax, soon afterwards disbanded in disgrace. The fighting continued until Domitian’s death in 96.
In 101-105, the warlike Emperor Trajan finally conquered the Dacians, reducing their lands to a Roman province. In 107, Trajan sent his general, Hadrian, to force the Iazyges to submit. In 117, Trajan died, and was succeeded as emperor by Hadrian, who moved to consolidate and protect his predecessor's gains. While the Romans kept Dacia, the Iazyges stayed independent, accepting a client relationship with Rome.
As long as Rome remained powerful, the situation could be maintained, but in the late second century, the Empire was becoming increasingly overstretched. In the summer of 166, while the Romans were tied down in a war with Parthia, the nomadic peoples north of the Danube, the Marcomanni, the Naristi, the Vandals, the Hermanduri, the Longobardi and the Quadi, all swept south over the Danube to invade and plunder the exposed Roman provinces. The Iazyges joined in this general onslaught in which they killed Calpurnius Proculus the Roman governor of Dacia . The Roman emperor Marcus Aurelius spent the rest of his life trying to restore the situation. In 170, the Iazyges defeated and killed Claudius Fronto, Roman governor of Lower Moesia. Operating from Sirmium (today Sremska Mitrovica, Vojvodina, Serbia) on the Sava river, Marcus Aurelius moved against the Iazyges personally. After hard fighting, the Iazyges were pressed to their limits.
But in 175, Avidius Cassius led a revolt in the East, interrupting the campaign. At this point, the leading king among the Iazyges, Zanticus, made peace with Marcus Aurelius, yielding up, it is said, 100,000 Roman captives. The Iazyges were also forced to provide the Romans with 8,000 cavalry to serve in the Roman army as auxiliaries. Some 5,500 of these were shipped off to serve in the Roman army in Britain; it is theorized they may have played a part in the development of the Arthurian legend. Marcus' victory was decisive in that the Iazyges did not again appear as a major threat to Rome.
Around 230, the Asding Vandals pushed in to the north of the Iazyges. The Vandals, and new Germanic tribal coalitions like the Alamanni and the Franks now became the Roman’s primary security concerns. But as late as 371, the Romans saw fit to build a fortified trading center, Commercium, to control the trade with the Iazyges.
In Late Antiquity, records become much more diffuse, and the Iazyges generally cease to be mentioned as a tribe.
They remained a distinct ethnographical group until today under the Hungarian name jászok (or jász in singular).
The only literary record of the Jassic language was found in the 1950s in the Hungarian National Széchényi Library on the backside of a diploma from 1443. It contains a short Jász-Latin vocabulary for monks in the newly founded monastery in Pilis mountains (N-W from Budapest), since the Jász people were settled in the area (e.g. the village Pilisjászfalu of today - a different area from the autonomous Jász territory around Jászberény).
The Iazyges' name is preserved in that of the Romanian city Iaşi (Jászvásár in Hungarian).
The connection between the Jazones (Yazones) and the Iazyges is disputed. Most Hungarian scholars claim that they were two different Sarmatian groups, and the Jazones are relatives of the Alans and the Ossetes. Others think that the Iazyges either migrated back east onto the steppes in the confusion of the Hun and Avar invasions of the 5th-7th centuries, or the Iazones were a fresh branch of the Iazyges that had never moved west before and remained throughout this period in what is now southern Russia. But based on the above diploma their languages should be very close.