Complex sentence: While in Sicily he was given the power to rule over the Sicilians as Praetors held imperium, which is the power to command an army and to quell any rebellions against Rome's administration in Sicily.
Split into simpler sentences: While in Sicily he was given the power to rule of the Sicilians as Praetors had imperium. Which was the power to command an army as he was allowed to quell any rebellions against Rome's administration in Sicily.