Politicians of the populares faction had been trying to extend citizenship to the Italians before (i.e. the Gracchi brothers) but such efforts were stopped by those of the senatorial optimates.

The socii eventually rebelled and set up an independent state with its own senate and other institutions, mostly mirroring the Roman state structure I recall.

I don't know how the war itself played out, but Rome eventually managed to bring the remaining rebels back into the fold by offering them citizenship in return.