The danger to Western Europe was averted. So crushing was the disaster that the Moors of Spain never again, during all the centuries that they ruled in the south, attempted to invade France. They retained, indeed, their hold of Narbonne and the districts bordering the northern slopes of the Pyrenees for some time longer (until 797), and even ventured upon foraying raids into Provence. But here their ambition ceased. The battle of Tours had once for all vindicated the independence of France, and set a bound to the Moslem conquests. Like the swelling tide of the sea, the Saracen hordes had poured over the land; and now, through the Hammerer of the Franks, a voice had spoken: "Hitherto shalt thou come and no further, and here shall thy proud waves be stayed." [Illustration: ARCH IN THE ALJAFERIA OF ZARAGOZA.] On the other hand, the kings of France were so deeply impressed with the courage of their Moslem neighbours, that, though they too delighted in occasional forays, once only did they attempt the subjugation of Spain. Charlemagne, the second Alexander, could not contemplate with composure the immunity of the Moslem power on the other side of the Pyrenees. As a good Christian he was pledged to extirpate the infidel; and, as an imperial conqueror, the existence of the independent kingdom of Andalusia was hateful to his pride. His opportunity came at last--when the accession of the first Spanish prince of the Omeyyad stock roused the hostility of some of the factions which were always prone to revolt in Spain. Charlemagne was invited to interfere and drive out the usurper. The Spanish chroniclers make Alfonso, King of the Asturias and heir of Pelagius,[6] summon the Frankish emperor to his aid; but there is more reason to believe that the invitation came from certain disappointed Moslem chiefs, who could not brook the authority of Abd-er-Rahmān the Omeyyad, and who were ready to submit even to the sworn enemy of Islam, rather than recognize the new ruler. The moment of their appeal was propitious; Charlemagne had just completed, as he thought, the subjugation of the Saxons; their chief Wittekind had been banished, and thousands of his followers were coming to Paderborn to be baptized. The conqueror's hands were thus free to turn to other schemes of victory. It was arranged that he should invade Spain, while the factious Moslem chiefs should make diversions in his favour at three different points. Fortunately for the newly-founded dynasty of Cordova, this formidable coalition came to naught. The allies in Spain miscalculated their time, and fell to blows with one another; and when Charlemagne crossed the Pyrenees in 777, he found himself unsupported. He began the siege of Zaragoza, when news was brought him that Wittekind had returned and raised the Saxons, who were again in arms, and had advanced as far as Cologne. There was nothing for it but to hurry back and defend his dominions. He rapidly retraced his steps, and the main