The Death of Muhammad(saws)
By now, the Prophet(saws) has united a larger part of Arabia than anyone had done before, and pagan cults died out as the number of converts to Islam increased everywhere. He led a Pilgrimage to the Ka'aba in the months of March, in a form according to Islamic beleif. Three months after returning to Madinah from what was later to be called his "Farewell Pilgrimage," he fell ill and died on 8th of June.
Abu Bakr(ra) as First Caliph
The Prophet's death took the Beleivers completely by surprise and created uncertainty about the future. No arrangements had been made for his succession, and it was the first main problem the Muslim community had to facewithout its ledaer. Abu Bakr(ra), the Prophet's father-in-law (through the marriage of Aisha) and his closest freind, was nominated to lead the prayers. He was two years younger than the Prophet(saws, and was elected the first Caliph or "successor[632-4]. However, it was understood that the Caliph would succeed only to the temporal role of the Prophet(saws) and that he could in no way claim those powers of Divine Revelation which belonged to the Prophet(saws) alone. The Caliph's relation to the religion would be no more than that of a guardian.
Many Arabs had come to associate the new religion with Muhammad(S) personally, and to them he had become the symbol of Islam. This fact was demonstarted in a number of tribes renouncing their allegiance soon after his death. Moreover, he did not live long enough to consolidate the Bedouin tribes into a nation, and loyalties were still divided on tribal lines. During his lifetime, he managed to establish the new community of Muslims on the basis of religion rather than tribal kingship, but the idea had not taken deep root, and nomadic instincts reasserted themselves. This presented itself as the most challenging problem to Abu Bakr(ra), threatening to destroy all that the Prophet(s) had achieved in a remarkably short time. Most of the early period of Abu Bakr(ra)'s reign was taken up in administrating the newborn Muslim state and at the same time dealing with these revolts against Madinah's rule in different parts of Arabia. In addition to these political and economic revolts(some tribes refused to pay Zakat, or poor tax), there were uprisings to become known as "wars of apostasy" led by "false prophets."
The sucess of the Prophet(s) mission had inspired many people with ambition, and several pretenders claiming to have a divine mission had arisen. The most notable of these was Musailima of the tribe of Hanifa in Yamama in central Arabia. He had a large following of his own tribe and, through his wife Saja, of the neighboring tribe of Tamim. Musailima, however, was quelled in january 633 by an army under Khalid ibn al-Walid. Other such revolts were also put down and eventually the whole of Arabia was united, by the prudence of Abu Bakr(ra) and the sowrd of Khalid, under the rule of the first Caliph.
The Accession of Yazdijird III
In the period of anarchy and civil war following the murder of Chosroes Parwiz by his son and successor Siroes in 628, there had been a number of quick successions to the Persian throne. As a result, the empure was badly shaken in the absence of any strong government. Soon after the Prophet(s) s death, Yazdijird III [d.651], a boy of fifteen, was made king of Persian, which included Iraq* and was one of the greatest power in the world.**
The new king, though a mere youth, managed to command the loyalty of most of the princes and generals and started to reorganize the state and the army. It was during his reign that the Muslims overthrew the Persian Empire, he being the last ruler of the Sasanid dynasty.
**Some historical accounts give the date of Yaazdijird's enthronement as two years later
The Surrender of Hira
Muthana ibn Haritha, a new convert to Islam after the Prophet(s)'s death, from the north-eastern Arab tribe called Bakr, was sent on a campaign in Iraq with Khalid ibn al-Walid and later took over the command. After the victory in the so-called Battle of the River of Blood at Ullais (near modern Samawah in Iraq), the two Muslim generals gave the enemy no respite and Hira was captured in May, with only a small and, in part, locally recruited force. Hira was a Persian outpost but largely inhabited by Arabs, and an important city in Iraq. It was also the first acquisition of the Muslims outside the Arabian Peninsula. Hira was spared military occupation and its Arab inhabitants were allowed to remain Nestorian Christains (this sect was bitterly hostile to the Monophysite form of Christainity prevailing among the Egyptians, Syrians, and Abyssinians) on the payment of a large sum of money.
Next year Khalid was suddenly ordered to go and take over the command of the Muslim armies in Syria who needed help, and Muthana was left behind in charge of the campaign on the Persian front.
The Military Expedition to Syria and Palestine
While the campaign in Iraq against the Sasnids was going on, at home major apostasy was stamped out by the revolting tribes being brought under control. Abu Bakr(ra) now turned his attention to the conquest of Syrian and Palestine on the Byzantine frontier which he regarded as more important. Through their commercial activities, the Arabs were more familiar with, and interested in, this region than Persia, which was largely unknown to them. Even the Prophet(s) in his lifetime showed interest in it and sent an expedition under his adopted son, Zayd ibn Haritha, to the Syrian broder in 629. But the vastly superior Byzantine army had no difficulty in crushing the raw soldiers of early Islam. Zayd died fighting, and the expedition ended in diaster.
In 633, Abu Bakr(ra) organized three armies of 3,000 soldiers eachg and put them under the command of Yezid ibn Abi Sufyan (his father, Abu Sufyan, the Makkan leader in many battles against the Muslims, was then governor of Najran and Hijaz), Shurahbil ibn Hasana and Amr ibn al-As (who was later to become famous for his conquest of Egypt in 640-2 and three times its governor). The first two columns entered Jordan, while Amr led his troops to south-eastern Palestine, and near Gaza he annihilated in February a small army under the local governor, Sergius.