Khalid was chosen by Abu bakr to lead the army as he had a reputation and led his troops from one victory to another. Abu bakr also collected and completed all the verses of the Qur’an
On his deathbed in 634 AC, Abu bakr appointed Umar b al-khattab as his successor.
'Umar was born into a respected Quraish family. When he grew up, Umar was proficient in this branch of knowledge as well as in swordsmanship, wrestling and the art of speaking. The most notable feature of umar’s caliphate was the vast expansion of Islam. Apart from Arabia, Egypt, Iraq, Palestine and Iran also came under the protection of the Islamic government. But the greatness of Umar himself lies in the quality of his rule. Umar gave his government an administrative structure. Departments of treasury, army and public revenues were established. Regular salaries were set up for soldiers. A population census was held. Elaborate land surveys were conducted to assess equitable taxes. New cities were founded. The areas which came under his rule were divided into provinces and governors were appointed. New roads were laid, canals were lug and wayside hotels were built. Provision was made for his support of the poor and the needy from public funds. He defined, by precept and by example, the rights and privileges of non-Muslims.
Umar went to the mosque to lead a prayer, a Magian named Abu Lulu Feroze, who had a grudge against Umar on a personal matter, attacked him with a dagger and stabbed him several times. Umar reeled and fell to the ground. When he learned that the assassin was a Magian, he said, “Thank God he is not a Muslim”.
Umar died in 644 CE.
After Umar’s assassination in 644, a six member committee appointed Uthman to be the next caliphate. In striving in the path of God and the expansion of Islam continued. Uthman's realm extended in the west to Morocco, in the east to Afghanistan, and in the north to Armenia and Azerbaijan. He also added Libya to the empire. During his caliphate a navy was organized, administrative divisions of the state were revised, and many public projects were expanded and completed. Uthman's compilation of the text of the Qur’an as revealed to the Prophet. Realizing that the original message from God might be inadvertently distorted by textual variants, he appointed a committee to collect the canonical verses and destroy the variant recensions. The result was the text that is accepted to this day throughout the Muslim world. Uthman ruled for twelve years. The first six years were marked by internal peace and tranquillity, but during the second half of his caliphate a rebellion arose. Uthman attempted to retain the unity of the empire by appointing members of his own clan, the Umayyads, to governorship, in particular, the governorship of his kinsman Muawiya was enlarged to include all of Syria and northern Iraq. These actions led to the charge of nepotism by Uthman’s detractors. A groups of rebels demanded he abdicate and some of the companions advised him to do so. The rebels broke into Uthman’s house and murdered him. Uthman breathed his last breaths in June 656 CE.
Uthman was murdered at the hands of discontented Egyptians in 656 CE and the notables of medina selected Ali b Abi Taliba s caliph. His failure to punish Uthman’s assassins quickly generated outrage. Ali had appointed new Governors but Muawiya declined to obey until Uthman’s blood was avenged. Civil war broke out under the leadership of Talha and Zubayr, two of Ali’s former supporters, and Aisha former wife of Muhammad (pbuh) and daughter of Abu Bakr. The rebellion was suppressed near Basra at the famous battle of the camel, because Aisha watched the battle from her palanquin of a camel. However, even though the era of Ali's caliphate was marred by civil strife, he nevertheless introduced a number of reforms, particularly in the levying and collecting of revenues. Ali also lost land already conquered by previous caliphates and didn’t expand the empire. A fanatical group called Kharijites, consisting of people who had broken away from Ali due to his compromise with Muawiya, claimed that neither Ali, the Caliph, nor Muawiya, the ruler of Syria were worthy of rule. In fact, they went so far as to say that the true caliphate came to an end with 'Umar and that Muslims should live without any ruler over them except God. They vowed to kill the two rulers, and assassins were dispatched in two directions. The assassins who were deputed to kill Muawiya did not succeed and were captured and executed, but Ibn-e-Muljim, the assassin who was commissioned to kill Ali, accomplished his task.
After Ali, Muawiya assumed the caliphate and thereafter the caliphate became hereditary, passing from one king to another
b) I think that all the caliphates were successful in dealing with the issues that they faced and secured the future of Islam. Abu bakr succeeded in conquering Syria and Iraq, he also dealt with the revolts and with tribes not willing to paying their taxes, he also completed the verses of the Qur’an. But was Abu bakr successful because the people need a leader to guide them. The Muslim community was not fully formed so they needed a guide which was Abu bakr. You could also say that being caliph for only two years would not be enough time to see the cracks. Umar was very successful in expanding the Muslim empire, and in developing the administrative structure. But some people would argue that he expanded the empire some much that it was too much for him and he could not handle it. But then again expanding the religion and empire was what needed to be done. Uthman had had six good and six bad years. He expanded the empire, made universal dialects of the Qur’an. But charged with nepotism and tried to suppress rebels and made the problem worse. He could have been influenced by his family since to make many of his decisions which made his last six years bad.
I feel that Ali was faced with the most difficult issues because, by the time he was caliph there was so much corruption and hatred in the community he had he most issues to deal with, much of the success was completed by the other caliphs. Ali had to face the most issues because of the previous caliphates. They had left problems that hadn’t been solved so Ali had to deal with them. You could argue and say that he was not successful in expand the Muslim empire but lost land, this was because he had to deal with Uthman’s problems and of his own. By the time Ali was caliph some of the Muslim lost faith in Islam and therefore rebelled against Ali. He had so much to deal with, that he had no time to expand the Muslim empire or the religion. All the Caliphs were successful because if they did not follow one after another then the future of Islam could have been destroyed, with no one to lead the Islamic community when they were undeveloped. The diversion of the Muslim community in to two groups was not down to the caliphates but down to the Muslims. They had the wrong idea. Shi’a opposed the caliphate system in favouring Ali. The Qurra never accepted Mu’awiya so Qurra formed a group called the Shi’a. All four caliphs were related to Muhammad in some way.