The diplomacy, as well as the strategy, of the Emperor was worthy of a far-seeing and cautious ruler. He kept the frontiers of the Empire assured by fortifications, wherever there was prospect of direct attack from the Danes or the Slavs, and by such means saw to it that the tribes bordering on the lines of defense were kept in awe and reduced to a state of dependence. In other places where the more distant Avars and the Bulgars might ultimately give trouble, the Emperor had taken care to come to a friendly arrangement with the Eastern Empire for mutual protection. This understanding did not, it is true, prevent friction between the two powers on the Adriatic Sea, where on several occasions the armies had met to decide their differences by arms. But on neither side was there any intention of developing consistent schemes for conquering the territory of the rival emperor. Disturbances were local and the border population was itself uncertain in allegiance, and ready to accept the guidance of its interests in determining the direction of its loyalty. This kind of hesitancy was not found in Italy, which remained inviolably faithful to Charles’ rule. The rulers of Constantinople had no time nor inclination to repeat the experiment of Justinian and Constans during the reign of Charles; they were weakened by serious difficulties of their own, due to disputed succession and religious conflict, and to the need of constant watchfulness against Moslem aggression. It was fortunate for both empires that the Saracens were not united. This was the most decisive factor, indeed, of the history of this period. The East was freed from the type of attack which had kept Leo the Isaurian constantly on the defensive, and which only his high military talents were able to cope with, while in the West the inability of the Moslems to act together made it possible for Charles to expand his territory and to give the time needed for internal development in the consolidation of his rule. It was one of the most permanent results of the activity of Charles as a conqueror that in the Spanish peninsula he strengthened not only the natural position of the petty and struggling Christian kingdoms, but by his personality made the ideal of a Christian ruler respected there, and so assured for the Christians in Spain a future which could be realized only when they had lived down their particularism and recognized the value of solidarity. But the wider field of armed conflict for the peoples included in his realm would have meant little, if it had not been accompanied by opportunities for real social progress.