When the Romans wouldn't grant the Italians equality, most of the allies attempted to secede, although Latium and northern Campania remained loyal to Rome. The rebels made their headquarters at Corfinium, which they renamed Italia. Poppaedius Silo headed the allied Marsic troops and Papius Mutilus headed the Samnites, altogether about 100,000 men.
The Romans divided their roughly 150,000 men under the 2 consuls of 90 B.C. and their legates. The Romans in the north were headed by P. Rutilius Lupus, with Marius and Cn Pompeius Strabo (Pompey the Great's father under whom Cicero served) under him. L. Julius Caesar had Sulla and T. Didius under him, in the south.
Rutilius was killed, but Marius was able to defeat the Marsi. Rome fared worse in the south, although Papius Mutilus was defeated by Caesar at Acerrae. The Romans made concessions after the first year of war.
The lex Julia gave Roman citizenship to some -- possibly all Italians who stopped fighting or just those who had remained loyal.
Next year, in 89 B.C., the Roman consuls were Strabo and L. Porcius Cato. They both went north. Sulla headed the Campanian forces. Marius had no commission despite his successes in 90. Strabo defeated 60,000 Italians near Asculum. The capital, "Italia", was abandoned. Sulla made progress in Samnium and captured the Italian HQ at Bovianum Vetus. The rebel leader Poppaedius Silo regained it, but it was defeated again in 88, as were other pockets of resistance.
Supplemental laws gave the franchise to the remaining Italians and people of the Italian regions of Gaul by 87. There was still a grievance, though, since new citizens were not equitably distributed among the 35 tribes of Rome.
H.H. Scullard: From the Gracchi to Nero.