Tradition rightly imposed upon Aetius the defence of Italy at the expense as it were of Cisalpine Gaul; it insisted that Cisalpine Gaul was to be the scene of the encounter. He determined to hold the line of the Po as he had held the line of the Loire; there was no need to be doubtful of his success. Already so many Barbarian invaders had found destruction in the immensity of that great plain. Nevertheless Aetius reinsured himself and Rome; he reinsured himself with Constantinople. It was no longer Theodosius the Calligrapher who sat on the Eastern throne, but Marcian the soldier. To him Valentinian sent ambassadors; Marcian heard them and promised an army. If, then, Aetius could lure Attila on far enough, but not too far for the safety of Italy, if he could hold him in the Cisalpine Plain, Marcian coming into Pannonia would be in time to cut off his retreat, and so at last the Hun would be utterly destroyed, and the bones of his great host might bleach beside the rivers of Lombardy. There at any rate we have the best explanation of what followed. Before the winter was over, the winter of 451-452, Attila was already moving south-west out of Barbary over the Danube, and at last by the great Roman road through Pannonia, crossing the Julian Alps as Alaric had done before him to cross the Isonzo, to lay siege to the first great Italian fortress, then perhaps, save Ravenna, the strongest place in all Italy, Aquileia, the capital of the province of Venetia. The walls of this mighty stronghold which was some sixty stadia from the sea were washed by the rivers Natiso and Turrus. I say it was, save Ravenna, the strongest place in Italy. It had been made so about the end of the fourth century, but it had much longer ranked third among Italian fortresses, only outstripped by Milan and Capua. Though set in the plain it was so strongly held with walls and towers that it enjoyed the reputation of being impregnable. Both Alaric and Radagaisus had passed it by; in the early spring of 452 Attila laid siege to it. For three months he laboured in vain; no engine he possessed, no contrivance he could command, no labour he could compel, were enough to break those Roman walls and to batter down the gates of this virgin fortress. He hoped to starve it out, but in three months the number of his armies, their depredations and ravages of the countryside began to tell far more against him than against the beleaguered city. Living on the country as he must do he was himself like to go hungry; moreover the spring heats in the marshy plains were already due, his hosts were discontented, they expected the loot of Italy, they began to remember the siege of Orleans and the battle of Châlons.