The Roman Republic (Fontana History of the Ancient World), Michael Crawford

Between the Sack of Rome by the Gauls in 390 BC and the middle of the second century BC, a part-time army of Roman peasants, under the leadership of the ruling oligarchy, conquered first Italy and then the whole of the Mediterranean. The loyalty of these marrauding heroes, and of the Roman population as a whole, to their leaders was assured by a share in the rewards of victory, rewards which became steadily less accessible as the empire expanded - promoting a decline in loyalty of cataclysmic proportions.