There was still, indeed, a gap in the line of frontier; how was the eastern end of the Alps to be protected from invasion? There, as we saw, the great rampart was lowest, and beyond it the barbarians were an unknown quantity. Here the river Danube eventually became the frontier, and was carefully connected with that of the Rhine; but this completion of the great work had to wait for half-a-century, and in the meantime luckily no inroad was made or threatened. It was Tiberius, afterwards emperor, another great soldier, with an army almost as devoted to its general as that of Cæsar who after long steady effort planted the Roman power firmly in this region. From a military point of view the Roman Empire, and therefore Western civilisation as a whole, owed its very existence for centuries to Pompey, Cæsar and Tiberius, with their splendidly trained armies and their skilful engineers. All this work of conquest and settlement was not the work of the State, or due to the old civic sense of duty and discipline; it was the work of the armies, due to their good discipline, and to their loyalty to their leaders. This being so, it was of course only natural that the armies and their leaders should claim to control the action and policy of an enfeebled State, as Sulla had already claimed it. This is really, put in a very few words, the secret of the Roman imperial system that was to come; so, too, in England, in Cromwell’s time, the State passed into the hands of the army, because that army (though in our case but for a short time) represented the best instincts and purposes of the nation. But the question of the moment was whether the commander of one of these Roman armies could so identify himself and his soldiers with the State and its true interests, as to become the means of establishing a sound and efficient government for the Mediterranean world. Sulla had failed so to identify himself: he had neither knowledge enough nor sympathy enough. The chance had been open to Pompey when he returned from the East in 62, but he had disbanded his army and declined it; he was in many ways a valuable man, but he was not the stuff that real statesmen are made of. After the long war in Gaul the chance was open to Cæsar, and he accepted it without hesitation.