In 91 BC the moderate members of the senate allied themselves with Livius Drusus (the son of that Drusus who had been used to undermine Gaius Gracchus' popularity in 122 BC) and aided him in his election campaign. If the honesty of the father is open to doubt, that of the son is not. As tribune he proposed to add to the senate an equal number of equestrians, and to extend Roman citizenship to all Italians and to grant the poorer of the current citizens new schemes for colonization and a further cheapening of the corn prices, at the expense of the state.
Though the people, the senators and the knights all felt that they would be conceding too many of their rights for too little. Drusus was assassinated.
Despite his eventually loss of popularity his supporters had stood by Drusus loyally. The opposition Tribune of the People, Q. Varius, now carried a bill declaring that to have supported the ideas of Drusus was treason. The reaction by Drusus' supporters was violence.
All resident Roman citizens were killed by an enraged mob at Asculum, in central Italy. Worse still, the 'allies' (socii)of Rome in Italy, the Marsi, Paeligni, Samnites, Lucanians, Apulians all broke into open revolt.
The 'allies' had not planned any such rising, far more it was a spontaneous outburst of anger against Rome. But that meant they were unprepared for a fight. Hastily they formed formed a federation. A number of towns fell into their hands at the outset, and they defeated a consular army. But alas, Marius took led an army into battle and defeated them. Though he didn't - perhaps deliberately - crush them.
The 'allies' had a strong party of sympathizers in the senate. And these senators in 89 BC managed to win over several of the 'allies' by a new law (the Julian Law - lex Iulia) by which Roman citizenship was granted to 'all who had remained loyal to Rome (but this most likely also included those who laid down their arms against Rome).
But some of the rebels, especially the Samnites, only fought the harder. Though under the leadership of Sulla and Pompeius Strabo the rebels were reduced on battlefield until they held out only in a few Samnite and Lucanian strongholds.
Was the city of Asculum in particular dealt with severely for the atrocity committed there, the senate tried to bring an end to the fighting by conceding citizenship to by granting citizenship to all who laid down their arms within sixty days (lex Plautia-Papiria).