The religion of Islam is what the Prophet, may Allah bless him and his family, propagated. For he (the Prophet): "Does not speak out of desire. It is naught but revelation that is revealed." So the Prophet had the right to nominate the Muslim successor for the succession. That is because the succession is the high responsible authority for the lawful. Therefore al-Hasan b. 'Ali was the legitimate successor whether the people pledged allegiance to him or not.
The Apostle of Allah, may Allah bless him and his family, mentioned al-Hasan within the chain of the names of the twelve imams. The Prophet said many traditions concerning al-Hasan and his brother al-Husayn. The Sunni scholars have reported these traditions.  All the Shi'ite scholars have agreed on the report of these traditions. In other words the two parties (Sunna and Shi'a) have agreed on these traditions. Among these traditions are: "You (al-Hasan and al Husayn) are two Imams. Your mother has the intercession.  "This (i.e., al-Husayn) is an Imam, the son of an Imam, the brother of an Imam, and the father of the nine Imams. 
When Imam 'Ali got ill, he ordered his son (al-Hasan) to lead the people in prayer. When Imam 'Ali was about to die, he entrusted al-Hasan and said: "My son, you are the trustee of authority and of blood." Imam 'Ali made al-Husayn, Muhammad (b. al-Hanafiya), all his sons, the chiefs of his Shi'a (followers), and the members of his House bear witness for his bequest. Imam 'Ali gave al-Hasan the Book and the weapon, then he said to him: "My son, the Apostle of Allah, may Allah bless him and his family, has ordered me to entrust you and to give you my books and my weapon as the Apostle of Allah entrusted me and gave me his books and his weapons. Also he (the Prophet) has ordered me to order you that when death comes to you, give them (the books and the weapon) to your brother al-Husayn." Then Imam 'Ali turned to al-Husayn and said: "The Apostle of Allah has ordered you to give them to this son of you."  Then he took the hand of 'Ali b. al-Husayn and said: "The Apostle of Allah has ordered you to give them to your son Muhammad. Send him the greetings of the Apostle of Allah and my (greetings). 
All tradition books have mentioned this tradition through authentic reliable chains of narrations on the authority of the members of the House (Ahl alBayt), peace be on them. This tradition of 'Ali was appropriate for the expected conditions. The Imami Shi'a use this method to prove the Imamate. The Sunni scholars have reported successive Prophetic traditions through their methods. Also non- Sunni scholars have clearly reported the same traditions through their methods. They (the traditions) have limited the Imamate to twelve Imams who were all from Quraysh.  Also they some times mention their names Imam by Imam to the last one of them, who is the awaited Mahdi (the rightly guided one), with whom Allah will fill the earth with justice when it is full of oppression and injustice.
Moreover, there are personal texts which the imams said to nominate each other.
Then the supremacy of each imam in knowledge, acts, miracles, and outstanding merits underlines both kinds of the above- mentioned traditions.
As for the pledge of allegiance of people, it is not a condition in the imamate of the Imam. In other words, people should pledge allegiance to those whom the Prophetic traditions have underlined. The Imamis regard pledging allegiance to other than the Imam as a correct act. It (pledging allegiance) is not accepted from anyone but at the time of compulsion.
The conditions prevented the community from pledging allegiance to the nominated Imams but two of them, who were 'Ali and al-Hasan peace be on them.
After al-Hasan, the formal succession started. Such kind of succession was usurped through weapons or money. Namely, such kind of succession occurred as al-Ghazali said: "The people assumed the succession without any merit." 
It was better for the Muslims or the Muslim historians in particular to regard the end of al-Hasan's succession as the beginning of the time of the kingdom. So they would be able to know that time clearly through its aspects, its policies, and its non-Islamic deeds. If they (the Muslims and the Muslim historians) had done that, they would have maintained the ideal, Islamic beliefs which that ideal succession followed. For they followed the example of the Prophet, may Allah bless him and his family. Also they (they Muslims and the Muslim historians) would have protected Islam from those kings who imposed their succession on Muslims. Unfortunately, the Muslim historians have called those kings successors while they (the kings) were not worthy of this title. For this reason, the mentioned historians have wronged Islam though they wanted to do good for it.
I (the author) wonder: was it appropriate for the successor, who had to be like the Prophet in his words and acts, to perform the Friday prayer on Wednesday or to perform it again in the morning? Was it appropriate for him to do Islamically forbidden acts? In other words, was it appropriate for him to sell gold for that which is more than it in weight, to attribute the corrupt persons to the family of the Prophet, to murder the believing people, and to supply the unbelievers with money to wage war against his Muslim brothers, and the like? So why had the historians not called such persons kings instead of calling them successor? Just imagine those persons who succeeded Mu'awiya. Did they not belong to the tree which Allah has cursed in the Qur'an? Just imagine Yazid, Abd al-Malik, al-Walid, and the like!
All these acts should urge Muslims to support Islam. They should not attribute to the Islamic authority but to those ideal Imams who followed the example of the Prophet, may Allah bless him and his family.
As we have said that al-Hasan was the most similar person to the Apostle of Allah, may Allah bless him and his family, in form, manner, and nobility.  He had the visage of prophets and the glory of kings. He is the lord of the youth of Paradise in the hereafter. The lord in the hereafter must be a lord in this world. His grandfather the Apostle of Allah, may Allah bless him and his family, surnamed him the absolute lord.
We have known that al-Hasan was the noblest person in ancestry. He was the best of men in father, mother, uncle, aunt, grandmother, and grandfather, as Malik b. `Ajlan described him during the meeting of Mu'awiya.
As al-Hasan had these noble traits and a textual nomination, then why didn't the people pledge Allegiance to him? Why didn't they let him assume the high religious position (i.e., the succession)? If it is difficult for us to understand the Imamate and the succession through these excellent abilities and outstanding merits, then which quality can replace them all and help us understand the Imamate or the succession?
Al-Hasan, peace be on him, carne out to meet the people. He was indifferent to their attitude towards him. Thus he stood on the pulpit of his father peace be on him to praise him. He said: "There has died to night a man who was the first among the early (Muslims) in (good) actions. Nor did any later (Muslims) attain his level in (good) actions. He used to fight alongside the Apostle of Allah, may Allah bless him and his family, and protect him with his own life. The Apostle of Allah, may Allah bless him and his family, used to send him forward with his standard while Gabriel supported him on his right and Michael supported him on his left. He would not return until Allah brought victory through his hands. He, peace be on him, has died on this night on which Jesus, son of Mary, was taken up (to Heaven), on which Joshua, son of Nun, the testamentary trustee (wasr) of Moses, peace be on him, died. He has left behind him no gold and silver except seven hundred dirhams of his stipend ( `ata), with which he was intending to buy a servant for his family." 
This praise of al-Husayn is unique. Also his style is oratorical. We have never read such kind of style. For al-Hasan, as you see, has not mentioned the known qualities of the great late Imam. People used to hear these qualities during such attitudes when great men are praised. People used to praise great men to show their noble qualities such as knowledge, patience, eloquence, bravery, generosity, lineage, nobility, faithfulness, refusal, and the like. Among the great men was 'Ali, who astonished the praisers. Therefore why did al-Hasan, peace be on him, refrain from following the known method in praising the great ones? I (the author) wonder: Did the strong shock resulting from his father's death prevent him from that? In other words, al-Hasan was an eloquent orator. He was the son of the most eloquent person in oration. Therefore why did he not mention his father's known qualities? Did he intentionally use this style? So al-Hasan was the most eloquent orator in using appropriate words for such an occasion. Besides he was the longest of them in oration though his words were short.
Yes, al-Hasan praised his father in the manner which no one had used before him in history. He mentioned certain qualities to praise his father. The qualities were not appropriate for anyone in history except 'Ali. Namely, no one of great men has such qualities. Therefore they are Alid traits.
Al-Hasan mentioned the Divine qualities of his father. He wanted to indicate that there was no one like his father throughout history.
Imam 'Ali was a man. However, the early people were not like him, nor were the later ones. He was a human being. However, he was between Gabriel and Michael. His spirit was taken up (to Heaven) on the night on which Jesus was taken up (to Heaven), on which Moses died, and on which the Qur'an came down to the earth. He spent all stages of his life with a close angel, a sent prophet, and a sent down Book. He was with the Apostle of Allah. He was ready to sacrifice his life for him. So the noble qualities in this world are not important in comparison with the Divine ones. For this reason, al-Hasan did not mention the noble traits in this world when he praised his father.
Now, you may agree with me on this unique style which al-Hasan used to praise his father, peace be on them. Namely, he used the most eloquent praise and the most appropriate style for the late Imam at that time.
This is one of al-Hasan's oratorical attitudes that indicate his close relation to his father and his grandfather, may Allah bless them and their families. Al-Hasan, the Successor, peace be on him, delivered many sermons after his father's death. For the people pledged allegiance to him. Moreover, he faced many critical situations.
`Ubayd Allah b. `Abbas b. `Abd al-Muttalib, al-Hasan's cousin, stood by the pulpit in al-Masjid al-Jami` that was full of people. He was waiting for the people to stop weeping after al-Hasan's praise for his father, peace be on them.
Then `Ubayd Allah said the following words with his sounding inherited voice that boomed in the earth as it boomed in the sky. From the first day, he propagated the message of the sky in the earth: "This is your Imam. He is the grandson of your Prophet and trustee of authority of your Imam. So pledge allegiance to him. `With him Allah guides those who will follow His pleasure into the ways of safety and brings them out of utter darkness into light by His will and guides them to the right path.'"
Many people remembered the tradition of the Apostle of Allah, may Allah bless him and his family, till the day when Imam 'Ali died. The tradition was concerning the succession of al-Hasan after his father. So the people said: "How dear is he (al-Hasan) with us! How obligatory is his right on us! What appropriate is he for the succession!" Then they rushed willingly to pledge allegiance to him.
That (the pledge of allegiance) was on the twenty- first of the month of Ramadan, on the day when his father (peace be on him) died, in the year 40 A.H. 
In this manner, Kufa was the first to pledge allegiance to al-Hasan, peace be on him. Then Basrah and al-Mada'in pledged allegiance to him. Then all Iraqis pledged allegiance to him. Then Hijaz (Saudi Arabia) and the Yemen, headed by the great leader Jariya b. Qaddama, pledged allegiance to him. Then Persia (Iran) pledged allegiance to him with the help of its leader Ziyad b. `Ubayd. Moreover, the great figures of the Emigrants (muhajrin) and of the Supporters (Ansar) pledged allegiance to him. Generally speaking, no one refused to pledge allegiance to al-Hasan, peace be on him, but Mu'awiya and his followers. So Mu'awiya led his followers to the way of misguidance. Besides other persons adopted a neutral attitude, so they were called al-quad (the neutral).
The religious succession was achieved according to its general phenomenon through the free pledge of allegiance. This was the second pledge of allegiance in the history of the family of Muhammad, may Allah bless him and his family. So al-Hasan's Imamate came to the Muslims through the holy way in which Prophethood came to them half a century before. So al-Hasan's Imamate had a close relation to the Apostle of Allah, may Allah bless him and his family. For it is regarded as continuation for the Prophetic mission through which the people were guided. In other words, the new Successor (i.e., al-Hasan) was ideal materially and spiritually.
According to these outstanding merits, al-Hasan, peace be on him, was the most appropriate person for the succession as the poet said:
He obtained the succession because it was predetermined for him.
As Musa (Moses) came to his Lord due to predetermination.
After the people had pledged allegiance to Imam al-Hasan, peace be on him, he started his new time through his historical, eloquent speech. Through this speech he mentioned the outstanding merits of the members of the House (Ahl al-Bayt) and their clear right to authority. Then he told the people about the expected dangers from which Islam would suffer. The following is some of his eloquent speech: "We are the victorious party of Allah. We are the close family of the Apostle of Allah. We are the good, pure members of his House. We are one of the two valuable things (al-thaqalayn) which the Apostle of Allah left behind him in his community. The other is the Book of Allah, in which there are details for everything. `False shall not come to it from before it nor from behind it.' So reliance is on us in explaining it. We do not depend on doubts in explaining it. Rather we are certain of its facts. Therefore obey us, for our obedience is obligatory. It is as obligatory as the obedience to the Apostle of Allah. Allah, the Great and Almighty said: `O you who believe! Obey Allah and obey the Apostle and those in authority from you; then if you quarrel about any thing, refer it to Allah and the Apostle.' Then He said: `.And if they had referred it to the Prophet and to those in authority among them, those among them who can search out the knowledge of it would have known it.'"
In the end of his speech, al-Hasan, peace be on him said: "I warn you not to listen to the temptation of the Satan, for he is a clear enemy for you. Therefore you will be like his followers to whom he said: `No one of the people can overcome you this day, and surely I am your protector; but when the two parties came in sight of each other he turned upon his heels, and said: Surely I am clear of you, surely I see what you do not see.' So `On the day when some of the signs of your Lord shall come, its faith shall not profit a soul that did not believe before, or earn good through its faith.'" 
Then al-Hasan, peace be on him, got off his pulpit to regulate the governors, to appoint the commanders, and to take care of the affairs. 
Accepting the Succession
Some pedantic critics thought that al-Hasan, peace be on him, was hasty when he accepted the succession at that time when the people pledged allegiance to him. For that time was about to result in hardships.
To understand whether al-Hasan, peace be on him, was hasty or not according to this criticism, we must explain the reason why al Hasan, peace be on him, accepted the succession:
1. As it was a religious duty on the people to pledge allegiance to the nominated Imam, it was incumbent on the Imam, who had supporters, to accept the succession from the people.
Al-Hasan, peace be on him, had many supporters. For the people all over the Muslim countries gathered together to pledge allegiance to him. So he had no right to fail to accept the obligatory succession. For the situation was appropriate.
2. This criticism concerning the succession of al-Hasan, peace be on him, has resulted from a pure worldly viewpoint. Thus the researcher must take the religious viewpoint into account. For there is a great difference between the religious viewpoint and the worldly one. From here, this matter subject is victory, not loss, as we will explain it in its proper place. Although the pledge of allegiance to al-Hasan, peace be on him, resulted in hardships, the hardships were for the sake of Islam. Then who was better than al-Hasan, peace be on him, in bearing the hardships for Islam?
3. Al-Hasan had a high social rank among the Muslim leaders, noble lineage, and plentiful knowledge. Thus he was unable to abandon the people even if he wanted that intentionally, nor did the people abandon him even if he wanted to abandon them. Moreover, many violent events occurred successively in the Muslim society. Such events urged him to take on an attitude to enforce the right and to forbid the wrong, as his brother al-Husayn, peace be on him, did at his time.
If al-Hasan, peace be on him, had abandoned the people and turned away from their pledge of allegiance to him, and if the people had abandoned him and refrained from pledging allegiance to him, the oppressive rulers would have not left him. For they thought that he was a fearful ghost. That is because his high social rank, his noble ancestry, and his plentiful knowledge spread righteousness among the people. Meanwhile the people were indignant with that situation. Thus they opposed the rulers, and propagated the religion of Allah. At that time, the people did not find anyone better than the beloved grandson of the Apostle of Allah to refer to in their religious affairs. Apart from this, delegations came to al-Hasan, peace be on him, and told him that they were ready to oppose the Umayyad rulers to bring back the usurped right. This underlines the clear wrath that prevailed the Muslim society, then.  Of course, the authority of the oppressors was shaky as long as this prominent figure (i.e., al-Hasan) was among the people who referred to him in their religious affairs.
We must always remember that al-Hasan, peace be on him, was given poison to drink. Why did they (the Umayyad rulers) kill him while he made peace with them and left all this world for them? Does this not mean that they were afraid of him? For he would shake their authority and prevent the hearts of the people from inclining towards them. Besides, does this not indicate that the people refused to consult anyone concerning their religious affairs except him?
All these events occurred after the Peace Treaty. Still some groups of his followers and other than they have criticized him for his peaceful attitude with Mu'awiya.
I (the author) wonder: how would the Muslims have treated al Hasan, peace be on him, if he had refused the succession from the beginning? That is because they were longing very much for his succession. Accordingly, was it possible for al-Hasan, peace be on him, to be the ray of hope and shelter for the indignant and the opposers? Was it possible for his careful eyes to sleep and leave their world?
So which mind accepts that al-Hasan, peace be on him, was hasty when he accepted the succession?
The succession, in its origin, was the position of his father. So he (al-Hasan) and his brother (al-Husayn) inherited it. That is what Imam Musa b. Ja'far, peace be on them, said.
As for the disturbances which these critics have mentioned, they were created by the opposers in Kufa. In the meantime, no one of those disturbances harmed al-Hasan, peace be on him, as long as the people supported him. It is worth mentioning that every caliph or leader has opposers.
So accepting the succession was the most feasible idea in all circumstances. Rather, it was incumbent on al-Hasan, peace be on him, to accept the succession to maintain the public interest and to enforce the right.
 Al-Qunduzi, Yanabi` al-Mawadda, vo1.2, p. 440. Ibn al-Khashshab, Ta'rikh. Ibn al-Sabbagh, al-Fusul al-Muhimma. Al-Hafiz al-Kunji, al-Bayan. As'ad b. Ibrahim b. al-Hasan b. 'Ali al-Hanbali, al-Arba'iniya. Hafiz al-Bukhari (Khaja Barsa), Fasl al-Khitab.
 Al-Shibrawi al-Shafi'i, al-Ithaf bi Hub al-Ashraf (Egypt), p.129. Al Safuri al-Shafi'i, vol. 2, p. 148.
 Ibn Taymiya, al-Minhaj, vol. 4, p. 210.
 Al-Mas'udi, Hamish b. al-Athir, vol. 6, 61.
 'Usul al-Kafi, p. 151. Kashf al-Ghumma, p.159.
 In his book called `al-Sahih, vol.2, p. 119, Chapter: People were the Followers of Quraysh, Muslim has reported on the authority of Jabir b. Samra, who said: "I (i.e., Jabir) heard the Apostle of Allah, may Allah bless him and his family, saying: `The religion will last till the hour (i.e., the Day of Judgment) starts, and there will be twelve Imams over them (people), who are all from Quraysh.'" A similar tradition has been reported by: al-Bukhari, al-Tirmidhi, al-Hamidi, and the like.
 Farid Wajdi, Da'irat al-Ma'arif, vo1.3, p. 231.
 Al-Mufid, al-Irshad, p. 167. Al-Ya'qubi, al-Ta'rikh, vol. 1, p.201.
 One day Mu'awiya said to the noble people from Quraysh and others: "Tell me about the best person in father, mother, uncle, aunt, grandmother, and grandfather." So Malik b. `Ajlan stood up, pointed to al Hasan, and said: "There he is! His father was 'Ali b. Abu Talib. His mother was Fatima, the daughter of the Apostle of Allah, may Allah bless him and his family. His uncle was Ja'far al-Tayyar (the one who flies in the gardens). His aunt was Umm Hani' the daughter of Abu Talib. His uncle was al-Qasim the son of the Apostle of Allah. His aunt was Zaynab the daughter of the Apostle of Allah. His grandfather was Allah's Apostle. And His grandmother was Khadija bint (the daughter of) Khuwaylid." (al-Bayhaqi, vol. l, p.62).
 Al-Ya'qubi, Ta'rikh, vol. 2, p.190. Ibn al-Athir, vol. 2, p. 190. Maqatil al-Talibiyyin.
 Ibn Abu al-Hadid, vol. 4, p. 11. Other than b. Abu al-Hadid, has mention `Abd Allah in stead of his brother `Ubayd Allah.
In Chapter `Leadership and War,' we will mention that `Abd Allah was not in Kufa when the people pledged allegiance to al-Hasan, peace be on him.
 Hisham b. Hasan has reported this sermon. He said: "It (the sermon) is some of his (i.e., al-Hasan's) sermons after the pledge of allegiance to him. See Bihar al-Anwar, vol. 10, p. 99. Also see al-Mas'udi
 Most historians have reported this text.
 Al-Imama wa al-Siyasa