International background. When Islam appeared on the world stage, the then world was dominated by two powers, Byzantium in the east and Persia in the west. There were spells of war as well as peace between these two years. During the sixth century, Justinian (507-565 C.E.) was the emperor of Byzantium, while Anaushirwan (531-579 C.E.) was the emperor of Persia. Both of them were contemporaries and great rulers of all world fame. In Byzantium, Justinian was succeeded by Maurice, and in Persia, Anaushirwan was succeeded by Khusro Perwez (Chosroes II). Chosroes II was overthrown in a military coup in 590, and he had to seek refuge with Maurice, the Byzantine emperor. With the Byzantine help, Chosroes II was restored to the Persian throne. Maurice regarded Khusro as a son, and during the last decade of the sixth century the two countries forged strong links of friendship. In 602 C.E., there was a revolt against Maurice. Maurice was killed, and Phocas became the emperor. There was another revolt in 610 C.E. when Heraclius became the Byzantine emperor. After the death of Maurice, the friendship between the two countries was over. In the second decade of the seventh century, Chosroes II invaded the Byzantine territories. Syria and Jerusalem fell to the Persians in 614 C.E. The Persians carried away the Holy Cross from Jerusalem. The Persians next marched to Egypt and annexed it in 616 C.E. For some time, the Byzantines lay low, but by 622 C.E. the Byzantines were strong enough to launch an attack against Persia. In the battle of Issus in 622 C.E., the Persians suffered a defeat. Other battles were fought during 623-625 C.E, which were not conclusive. The decisive battle was fought on the banks of the Tigris near the city of Mosul in 625 C.E. when Persia surrendered and asked for terms. As a result of this reverse, there was a revolt against Chosroes II in 628 C.E., when he was killed by his own son Sheroyah. Sheroyah who ascended the Persian throne as Kobad II made peace with Heraclius. By the terms of the peace treaty Persia abandoned all the conquests that it had made earlier in the second decade of the seventh century. Sheroyah died within a year. After him there was complete anarchy in the Sassanian empire, and during the next four years, there were a dozen kings including, two women. The Byzantine Empire on the other hand enjoyed a measure of stability under Heraclius.
Arab buffer states. When the two empires of Persia and Byzantium expanded, these came to include territories populated by Arabs. As a matter of policy both the empires found it expedient to set up Arab buffer states at the periphery of their empires. In the sixth century, a Ghassanid Arab state was set up in Syria under Al Harith b Jabala. This state acknowledged the suzerainty of Byzantium. In the Persian Empire a Lakhmid state was set up in Iraq with the capital at Hira. The Lakhmids acknowledged the suzerainty of Persia. The Ghassanids and the Lakhmids were often at war with each other. When Islam appeared on the world stage, the position about these buffer Arab states was changed. In Syria after the death of their king Al Harith b Jabala the Ghassanid State split into fifteen principalities. In Persia Chosroes II did away with the Lakhmid State, and took over the territory under the direct rule of Persia. The policy of the Holy Prophet was to win over the border Arab tribes to Islam. It was with a view to this end that the campaigns of Muta and Tabuk were undertaken during the lifetime of the Holy Prophet. When Abu Bakr insisted on sending Usama's expedition to Syria, it was in continuation of the policy laid down by the Holy Prophet. With the disintegration of the Persian rule, there was a power vacuum in the coastal areas of east and south Arabia. Islam succeeded in filling up this vacuum. In Iraq, Islam had yet to make headway.
Campaigns of Abu Bakr. When Abu Bakr became the Caliph in 632 C.E., Islam was threatened with disintegration. Within a year, Abu Bakr was strong enough to attack the Persian Empire on the northeast and the Byzantine Empire in the northwest. These were giant empires with history extending over hundreds of years. They had large resources at their disposal. But yet against the Arab hordes the Persian and the Byzantine forces were not able to take a stand. In Iraq the Muslim forces gave blows after blows to the Persian armies. In Syria the same story was repeated and the Byzantine forces in spite of the superiority in strength and vastness of resources could not withstand the Muslim forces. The story of the victory of the Muslim armies in Iraq and Syria read very much like a tale of the Arabian Nights, too difficult to believe, but yet an established fact of history. In this respect, Professor Hitti observes as follows in his History of the Arabs: "If someone in the first third of the seventh Christian century had the audacity to prophesy that within a decade some unheralded, unforeseen power from the hitherto barbarians and little known land of Arabia was to make its appearance, hurl itself against the only two powers of the age, fall heir to the one-the Sassanids, and strip the other, the Byzantine of its fairest provinces, he would undoubtedly be declared a lunatic. Yet that was what happened."