Excerpts from Books and Wikipedia
"The Social War (from socii ("allies"), thus Bellum Sociale; also called the Italian War, the War of the Allies or the Marsic War) was a war waged from 91 to 88 BC between the Roman Republic and several of the other cities in Italy, which prior to the war had been Roman allies for centuries." [Wikipedia]
"First, war was declared on Rome in 91 BCE by a coalition of Italian allies, or socii (hence the quaint, and deceptively harmonious, modern title of Social War). Within a couple of years the Romans more or less defeated the allies, and in the process gave most of them full Roman citizenship. Even so, the death toll – among men who had once served side by side in Rome’s wars of expansion – was, according to one Roman estimate, around 300,000. Exaggerated as that figure may be, it still points to casualties on a scale not far from that of the war against Hannibal." [SPQ, p. 217]