Omar was succeeded by Ali, the husband of Mohammed’s daughter, Fatima, but a quarrel broke out upon a point of Moslem doctrine and Ali was murdered. After his death, the caliphate was made hereditary and the leaders of the faithful who had begun their career as the spiritual head of a religious sect became the rulers of a vast empire. They built a new city on the shores of the Euphrates, near the ruins of Babylon and called it Bagdad, and organising the Arab horsemen into regiments of cavalry, they set forth to bring the happiness of their Moslem faith to all unbelievers. In the year 700 A.D. a Mohammedan general by the name of Tarik crossed the old Gates of Hercules and reached the high rock on the European side which he called the Gibel-al-tarik, the Hill of Tarik or Gibraltar. Eleven years later in the battle of Xeres de la Frontera, he defeated the king of the Visigoths and then the Moslem army moved northward and following the route of Hannibal, they crossed the passes of the Pyrenees. They defeated the Duke of Aquitania, who tried to halt them near Bordeaux, and marched upon Paris. But in the year 732 (one hundred years after the death of the prophet,) they were beaten in a battle between Tours and Poitiers. On that day, Charles Martel (Charles the Hammer), the Frankish chieftain, saved Europe from a Mohammedan conquest. He drove the Moslems out of France, but they maintained themselves in Spain where Abd-ar-Rahman founded the Caliphate of Cordova, which became the greatest centre of science and art of mediæval Europe. [Illustration: THE STRUGGLE BETWEEN THE CROSS AND THE CRESCENT] This Moorish kingdom, so-called because the people came from Mauretania in Morocco, lasted seven centuries. It was only after the capture of Granada, the last Moslem stronghold, in the year 1492, that Columbus received the royal grant which allowed him to go upon a voyage of discovery. The Mohammedans soon regained their strength in the new conquests which they made in Asia and Africa and to-day there are as many followers of Mohammed as there are of Christ. CHARLEMAGNE HOW CHARLEMAGNE, THE KING OF THE FRANKS, CAME TO BEAR THE TITLE OF EMPEROR AND TRIED TO REVIVE THE OLD IDEAL OF WORLD-EMPIRE The battle of Poitiers had saved Europe from the Mohammedans. But the enemy within—the hopeless disorder which had followed the disappearance of the Roman police officer—that enemy remained. It is true that the new converts of the Christian faith in Northern Europe felt a deep respect for the mighty Bishop of Rome. But that poor bishop did not feel any too safe when he looked toward the distant mountains. Heaven knew what fresh hordes of barbarians were ready to cross the Alps and begin a new attack on Rome. It was necessary—very necessary—for the spiritual head of the world to find an ally with a strong sword and a powerful fist who was willing to defend His Holiness in case of danger.