Speedily the plains about Medina were covered with the encampments of the chiefs who had responded from all Arabia, in hope of the rich booty to be had from conquered cities and provinces. From the brow of a hill, Abu Beker reviewed the army on the point of departure. His heart swelled with pride, as he gazed on the passing multitudes; the glittering arms; the squadrons of horsemen; the lengthening line of camels, and called to mind the handful of men that followed Mohammed a fugitive from Mecca. Scarce ten years had elapsed, and now a mighty host assembled at the summons of his successor, and distant empires were threatened by the sword of Islam. He lifted up his voice and prayed God to make these troops valiant and victorious. Then giving the word to march, the tents were struck, the camels laden, and in a little while the army poured forth in a long, continuous train over hill and valley. The "Scourge of God" was let loose against the nations. Before long an immense cavalcade of horses, mules and camels came pouring in with the booty from the first victory over a body of troops sent by the Emperor Heradius to observe them and harass their march. Soon four armies were in the field; Jerusalem and Damascus were doomed and fate hastened on its march to Persia in the person of Khaled, "The Sword of God" with an army of ten thousand men. He besieged the city of Hira; stormed its palaces; slew the king in battle; subdued the kingdom; imposed on it an annual tribute of seventy thousand pieces of gold; the first tribute ever levied by Moslems on a foreign land, and sent the same to Medina. City after city fell before him. Nothing seemed able to withstand his arms. Planting his victorious standard on the banks of the Euphrates, he wrote to the Persian monarch calling upon him to embrace the faith or pay tribute. "If you refuse both, I will come upon you with a host who love death as much as you do life." But meantime partial defeat had discouraged the leaders of the armies in Syria, and the caliph summoned Khaled to the command of the northern armies. Leaving the army in Persia under the command of a tried and trusty general, Khaled with an escort of fifteen hundred men spurred across the Syrian borders to join the Moslem host about to besiege the Christian city of Bosra. It was on the Syrian frontier, a walled city of great strength and wealth, that could at anytime put twelve thousand men into the field. After two days of furious battle the city was taken by treachery, many were massacred, and the survivors were compelled to pass under the yoke.