Please answer a question about the following article about Roman Republic:

Rome's military confederation with the other peoples of the Italian peninsula meant that half of Rome's army was provided by the Socii, such as the Etruscans, Umbrians, Apulians, Campanians, Samnites, Lucani, Bruttii, and the various southern Greek cities. Polybius states that Rome could draw on 770,000 men at the beginning of the Second Punic War, of which 700,000 were infantry and 70,000 met the requirements for cavalry. Rome's Italian allies would be organized in alae, or wings, roughly equal in manpower to the Roman legions, though with 900 cavalry instead of 300.

Where did Rome receive half of their army from?
the Socii