Imam Hasan a.s. argued his rights to the caliphate on the grounds that the authorityof the caliphate stems from the Prophet Mohammed s.a.w.w.who was the most excellent and the best of men on earth and through whose guidance the Arabs found light while they were deep in darkness and attained honour and glory while they were disgraced, and that Imam Hasan a.s. was the nearest to the Prophet Mohammed s.a.w.w. in blood and relationship.
Imam Hasan a.s. then used his father's argument, which the latter had advanced against Abu Bakr after the death of Prophet Muhammad s.a.w.w., that if Quraysh (Mecca's resident and belonging to a particular tribe)could claim the leadership over the Ansar (residents of Medina)on the grounds that the Prophet belonged to Quraysh, then the members of his family, who were the nearest to him in every respect, were better qualified for the leadership of the community.
In the last part of his letter Imam Hasan a.s. wrote: "We were shocked to see that some people snatched away our right from us even though they were men of excellence, virtues, and merits, and were the forerunners in Islam [reference to the first three caliphs]. But now what a great astonishment and shock it is to see that you, O Mu'awiya, are attempting to accede to a thing which you do not deserve. You do not possess any known merit in religion (deen), nor have you any trace (athar) in Islam which has ever been praised. On the contrary, you are the son of the leader of the opposition party from among the parties (hizb min al-ahzab) [a reference to the "confederacy" which under Mu'awiya's father, Abu Sufyan, made the last united effort to crush Medina]; and you are the son of the greatest enemy of the Prophet s.a.w.w. from among Quraysh... so give up your persistence in falsehood (batil) and enter into my homage as other people have done, for you are certainly aware of the fact that I am far more entitled to the caliphate than you in the eyes of God and all worthy people. Fear God, restrain yourself from rebellion and from shedding the blood of the Muslims; for, by God, there would be no good for you to meet your Lord with the responsibility of the blood of the Muslims."
Mu'awiya's detailed reply to Imam Hasan a.s. is even more interesting, especially since he used the argument used by 'Umar b. al-Khattab' against Mola Ali a.s.. Writing to Imam Hasan a.s., Mu'awiya argued: "Whatever you said about the excellence and merits of the Prophet, he was indeed the most excellent among all men before and after him, past or present, young or old. Indeed God had chosen Muhammad for His message, and through him we received guidance, were saved from destruction, and came out from darkness and error."
"You have mentioned the death of the Prophet and the dispute which took place among the Muslims at that time. In this you are clearly making accusations against Abu Bakr, 'Umar, and Abu 'Ubayda, and against those virtuous men among the Muhajirun and Ansar. I hate this accusation against the people whose actions, according to us and other people, were beyond doubt and reproach."
"When this community had some disagreements after the Prophet concerning the leadership, it was not ignorant of your family's merits, your priority, and your close relationship to the Prophet; and the community was also not unaware of your exalted place in Islam and your qualifications in it. But the community saw that this thing [the caliphate] would be better placed among Quraysh in general and they therefore selected Abu Bakr. This is what the people thought best in the interest of the community. You are asking me to settle the matter peacefully and surrender, but the situation concerning you and me today is like the one between you [your family] and Abu Bakr after the death of the Prophet. Had I believed that you had a better grasp over the subject people than I do, that you could protect the community better than I, and you were stronger in safeguarding the properties of the Muslims and in outwitting the enemy than I, then I would have done what you have asked me. But I have a longer period of reign [probably referring to his governorship], and am more experienced, better in policies, and older in age than you. It would therefore be better for you not to insist on what you have asked me; if you enter into obedience to me now, you will accede to the caliphate after me."
Mu'awiya's letter is significant in that it gives a clear idea of the direction Muslim polity was henceforth opting to adopt openly. Mu'awiya's arguments for his claims to the caliphate manifest those guidelines and the principles by which the question of the caliphate had been previously decided in the case of the first three caliphs, and he claimed that the same considerations must remain the deciding factors now and in the future. To him it was the interest of the state and the profane aspects of the community which must decide the question of the leadership. Muawiya did not deny Imam Hasan a.s. exalted position in relation to the Prophet Mohammed s.a.w.w. and his superior place in Islam, but claimed that this was not the criterion for the leadership of the community.The qualifications for the office, according to Mu`awiya's arguments, were personal power and strength, ability in political affairs and administration, expansion of the empire, and ability to defend the Muslims and rule the subject effectively. In this way,
Mu'awiya made explicit what had been so far implicit: the separation between political and religious principles, which was henceforth permanently established.
Thus, in due course, the majority of the Muslims placed the religious leadership in the totality of the community (Jama`a), represented by the 'ulama', as the custodian of religion and the exponent of the Qur'an and the Sunna of the Prophet, while accepting state authority as binding. They came to be known as the Sunnis.
A minority of the Muslims, on the other hand, could not find satisfaction for their religious aspirations except in the charismatic leadership from among the people of the house of the Prophet Mohammed s.a.w.w., the Ahl al-Bayt, as the sole exponents of the Qur'an and the Prophetic Sunna, although this minority too had to accept the state's authority. This group was called the Shi'a.
Sourced above from book: The Origins and Early Development of Shia Islam by SHM Jafri