A few traces of a more humane spirit may, it is true, be discovered. Plato had advocated the liberation of all Greek prisoners upon payment of a fixed ransom,(531) and the Spartan general Callicratidas had nobly acted upon this principle;(532) but his example never appears to have been generally followed. In Rome, the notion of international obligation was very strongly felt. No war was considered just which had not been officially declared; and even in the case of wars with barbarians, the Roman historians often discuss the sufficiency or insufficiency of the motives, with a conscientious severity a modern historian could hardly surpass.(533) The later Greek and Latin writings occasionally contain maxims which exhibit a considerable progress in this sphere. The sole legitimate object of war, both Cicero and Sallust declared to be an assured peace. That war, according to Tacitus, ends well which ends with a pardon. Pliny refused to apply the epithet great to Cæsar, on account of the torrents of human blood he had shed. Two Roman conquerors(534) are credited with the saying that it is better to save the life of one citizen than to destroy a thousand enemies. Marcus Aurelius mournfully assimilated the career of a conqueror to that of a simple robber. Nations or armies which voluntarily submitted to Rome were habitually treated with great leniency, and numerous acts of individual magnanimity are recorded. The violation of the chastity of conquered women by soldiers in a siege was denounced as a rare and atrocious crime.(535) The extreme atrocities of ancient war appear at last to have been practically, though not legally, restricted to two classes.(536) Cities where Roman ambassadors had been insulted, or where some special act of ill faith or cruelty had taken place, were razed to the ground, and their populations massacred or delivered into slavery. Barbarian prisoners were regarded almost as wild beasts, and sent in thousands to fill the slave market or to combat in the arena.