A noticeable space has remained among these gaps in history. The references we have are not able to fill this space with studies that suite those events.
Up to this topic, we have understood that Mu'awiya violated all the conditions with which he bound himself.
We have known, up to here, that Mu'awiya did not take care of the Peace Treaty with five items. In other words his care did not suite those covenants, agreements, and the oath with which he bound himself.
When Mu'awiya took the reins of government, he did not act according to the Book of Allah, the Sunna (i.e., practices) of His Apostle, and the practices (Sira) of the Orthodox Caliphs, nor did he leave the authority after him to the Consultative Committee or to the Owner of the right (i.e., al-Hasan), nor did he refrain from cursing 'Ali, nor did he give (al-Hasan) the land taxes, nor did he give security to 'Ali's Shi'ites and companions. In spite of these stipulations and pledges, he increased cursing 'Ali to the extent that he ordered his orators to curse him everywhere. Moreover, he committed horrible crimes against the Shi'ites of 'Ali:
Thus the first head to be roamed in Islam was from them. He (i.e., Mu'awiya) ordered it to be roamed.
The first man to be buried alive in Islam was from them. He ordered him to be buried alive.
The first woman to be imprisoned in Islam was from them. He ordered her to be imprisoned.
The first patient martyrs were from them. It was he who killed them.
Mu'awiya violated all the items of the Peace Treaty, broke his strong oath, and opposed his certain pledges which he made before Allah, the Most High.
Therefore I (i.e., the author) wonder: Is this the religious succession?
Mu'awiya protected himself from the last stipulation in the Peace
Treaty, for it was the most sensitive of all the stipulations and the strongest of them all. If he had broken this condition, then he would have challenged the Qur'an openly, and the Apostle of Allah, may Allah bless him and his family, directly.
Thus Mu'awiya was patient towards this condition for eight years. Then he was unable to bear it. For his Umayyad lineage moved him to make his children inherit the authority. In this way he underlined that he was b. (the son of) Hind and Abu Sufyan.
Wasn't there a great difference between b. Abu Sufyan (i.e., Mu'awiya) and the Apostle of Allah? Wasn't there a great difference between b. Hind (i.e., Mu'awiya) and the Book of Allah?
Then it (i.e., the pledge of allegiance to his son Yazid) was the first malice that came to the Arabs, as b. `Abbas, may Allah be pleased with him, said.
Rather it was the first malice that came to the people, as Abu Ishaq al-Subay'i, may Allah be pleased with him, said.
It (i.e., the succession to authority) was, in its nature, -the farthest of all the items of the Peace Treaty from treason. Also it was, in its conditions, the worthiest of them all of care. It was the most horrible crime in Mu'awiya's life full of crimes. For it (i.e., the Peace Treaty) occurred when the soldiers were disarmed, the standard war wrapped, and the enemy bound himself to fulfill the conditions.
Neither in Medina (the homeland of al-Hasan, peace be on him) nor among the members of the House (Ahl al-Bayt, peace be on them) nor among the Shi'ites of al-Hasan and his relatives was a justification to make Mu'awiya fear for his life in this world.
Therefore what is this deception? What is the excuse?
Weren't those covenants, agreements, and oath the strongest and the most certain words in the dictionaries of the Arabic language?
I (i.e., the author) wonder: Shall we apologize on behalf of Mu'awiya as those honorable ones who are attributed to Islam apologize on behalf of his son Yazid who killed al-Husayn, the grandson of the Apostle of Allah, the best blessing and peace be on him and his grandfather, when they said: "He (i.e., Yazid) was a self conceited young man. The monkeys distracted him. Wine and dissoluteness controlled him?"
Therefore where was the experience of Mu'awiya and his claimed cleverness? Where was his old age and his experience in the matters?
This deed of the father (i.e., Mu'awiya) urged the son (i.e., Yazid) to follow his father's example to achieve his ambitions. Thus they cooperated with each other to accomplish the greatest crime in the history of Islam. That was the killing of the two Lords of the youth of Heaven, the two ones of whom there was no third. Also they cooperated with each other to cut the only means to which the progeny of the Apostle of Allah, may Allah bless him and his family, was limited. The crime, in this meaning, is direct killing to the historical continuity of the life of the Apostle of Allah, may Allah bless him and his family!
Yes, nevertheless, the two killers were two caliphs in Islam!
What a great loss Islam suffered from when its caliphs were of such a kind!
It was the claimed cleverness that urged Mu'awiya to follow such a kind of killing, which his son Yazid was unable to accomplish. Thus this (i.e., Yazid) was "a self-conceited young man and that (i.e., Mu'awiya) was experienced in managing the affairs!
If Abu Sufyan had lived for a longer time, he would have been sure that these two sons of his (i.e., his son Mu'awiya and his grandson Yazid) were able to achieve the game, which he hoped for the children of his father.
Accordingly, Mu'awiya ordered Marwan b. al-Hakam  to convince Ju'da bint al-Ash'ath b. Qays al-Kindi, who was one of the wives of al-Hasan, peace be on him, to give poison, which was a drink of honey mixed with Rome water, to drink. If al-Hasan died, Mu'awiya would marry her to his son Yazid, and would send her a hundred thousand dirhams.
As Ju'da was the daughter of al-Ash'ath b. Qays, the known hypocrite who became Muslim twice and there was atrocious apostasy between them, she was the nearest of all people to accept this abominable bargaining.
Imam Ja'far b. Muhammad al-Sadiq, peace be on him said: "Indeed al-Ash'ath took part in killing the Commander of the faithful (i.e., Imam 'Ali), peace be on him. His daughter Ju'da poisoned al-Hasan. Moreover, his son Muhammad took part in killing al-Husayn."
I (i.e., the author) say: In this manner Mu'awiya accomplished what he wanted.
Through this plot Mu'awiya was able to control the affairs of all the community. Thus he subjected the community to many disasters. Moreover, he filled himself and his children with spites so that they
were liable to many battles and coups.
Through this plot Mu'awiya was able to abolish all items of the Peace Treaty.
When al-Hasan, peace be on him, was about to die, he said:" Mu'awiya's drink (of poison) has acted upon (me). He has obtained his wish. By Allah, he has not fulfilled what he promised, nor has he been truthful in what he said." 
Marwan's letter concerning carrying out the evil plan came to Mu'awiya. So the latter said: "How wonderful! Al-Hasan took a drink of honey (mixed with) Rome water, so he died." 
Then Mu'awiya was unable to restrain his happiness when al Hasan, peace be on him, died.
"Mu'awiya was at al-Khadra'. Thus he exclaimed: `Allah is Great!' The people of al-Khadra' exclaimed: `Allah is Great!' with him. Then the people of the mosque exclaimed: `Allah is Great' when the people of al-Khadra' did that. So Fakhta (Mu'awiya's wife) bint (daughter of) Qarza b. `Amr b. Nawfa b. `Abd Munaf came out of her wicket and said: `Commander of the faithful, may Allah delight you.' What has made you happy? 'The death of al-Hasan,' he said. `To Allah we belong and to Him is our return,' she said. Then she wept and said: `The Lord of the Muslims and son of the daughter of the Apostle of Allah, may Allah bless him and his family, died.' `By Allah, what you have done is excellent. He was so. He is worthy of weeping over him,' he said."
A part from this, b. Qutayba said: "When the news (of the death of al-Hasan) came to Mu'awiya, he showed rejoice and pleasure to the extent that he and those who were with him bowed down. `Abd Allah b. 'Abbas, who was then in Sham (Syria), heard of that, so he came to Mu'awiya. When he sat down, Mu`awiya said: `Bin `Abbas, al-Hasan b. 'Ali perished.' Thus b. 'Abbas said: `Yes, he perished. To Allah we belong and to Him is our return. I have heard that you showed rejoice and pleasure when he died. Indeed, by Allah, his body has not taken the place of your grave, nor has the decrease of (the period of) his death increased your life. He died while he was better than you. If we have been afflicted by him, then we had been afflicted by him who was better than him, his grandfather the Apostle of Allah, may Allah bless him and his family. Thus Allah comforted his disaster and appointed the best successor over us after him.
"Then b. `Abbas wept. Those who attended the meeting wept. Mu'awiya wept. The narrator said: `I have never seen weepers as many as those who were present on that day.' `How old was al-Hasan?' Mu'awiya asked. `Al-Hasan's birthday is very famous that everybody knows it,' replied b. `Abbas. He (i.e., the narrator) said: Mu'awiya kept silent for a while, and then he said: `Bin `Abbas, you have become the lord of your people after him.' `(I am not the lord of my people) as long as Allah has kept Abu `Abd Allah al-Husayn alive', said b. `Abbas." 
Al-Ya'qubi (Ta'rikh, vol. 2, p. 203.) has mentioned a description about the great sadness of the Kufans when they heard of the death of al-Hasan, peace be on him. Thus the leaders of the Shi'ites there held a meeting in the house of their chief Sulayman b. Sirt ak Khuza'i. Moreover, they condoled al-Husayn through a painful eloquent letter.
The news of the death of al-Hasan reached the ears of the people in Basrah which Ziyad b. Sumayya governed. So the people wept loudly. Thus Abu Bakra, Ziyad's half brother (on his mother's side), who was then ill in his house, heard that and said: "Allah has relieved him of much evil. The people have lost much good because of his death. May Allah have mercy on al-Hasan." 
His (i.e., al-Hasan's) brother Muhammad b. al-Hanafiya praised him when he stood by his holy body. The following is his praise: "Abu Muhammad (i.e., al-Hasan), may Allah have mercy on you. By Allah, indeed if your life was valuable, your death has undermined (us). The excellent soul was that through which your body lived. The excellent body is that which your shroud has covered. Of course, you are so. For you are the descendant of guidance, the ally of the pious, and the fifth (person) of those who were covered with the cloak (ashab al-Kisa ). The hand of the truth fed you. You grew in the lap of Islam. The two breasts of belief suckled you. You are good whether you are alive or dead. Therefore peace and Allah's mercy be on you even though we did not hate you nor did they doubt that the choice was for you. 
The texts denoting that Mu'awiya poisoned al-Hasan are very obvious in history. These texts have been mentioned in the following books:
1) al-Isti'ab by `Abd al-Bir al-Maliki,
2) al-Isaba fi Tamyiz al Sahaba by Ahmad Shahab al-Din al-`Asqalani,
3) Maqatil al-Talibiyyin by Abu al-Faraj al-Isfahani,
4) al-Furuq bayna al-Abatil wa al-Huquq by al-Shaybani,
5) Ta'rikh by al-Ya'qubi,
6) al-Tabaqat by b. Sa'd,
7) alAhdath by al-Mada'ini,
8) Ta'rikh Dimashq by Ibn `Asakir,
9) Asad al Ghaba fi Tamyiz al-Sahaba by Ibn al-Athir,
10) Muruj al-Dhahab by al Mas'udi,
11) Sharh Nahj al-Balagha by b. Abu al-Hadid,
12) Tanzih al Anbiya' by al-Murtada,
13) al-Amali by al-Tusi,
14) al-Diwan by al-Sharif al-Radi,
15) al-Mustadrak by al-Hakim,
16) al-Irshad by al-Mufid,
17) Tadhkirat al-Khawas by al-Dhahabi,
18) Dala'il al-Imama by al-Tabari, ...
and the like.
In his book `al-Bidaya wa al-Nihaya Ibn Kathir said: "Al-Hasan died in the year 49 A.H. Ju'da bint al-Ash'ath poisoned him with what Mu'awiya had sent to her. He arranged for her to marry his son Yazid. Then he broke his promise."
In his book `al-Tabaqat,' b. Sa'd said: "Mu'awiya poisoned al Hasan several times."
Al-Mada'ini said: "Al-Hasan was given poison to drink four times."
In his book `al-Mustadrak', al-Hakim said: "Indeed al-Hasan b. 'Ali was given poison to drink several times. He was safe during these times except the last time when he died. Indeed he spewed his liver." 
Al-Ya'qubi said: "When death was close to al-Hasan, he said to his brother al-Husayn: `My brother, these are the last three times when I have been given poison to drink but I have never been given poison like this time. I will die on this day of mine. When I die, then bury me beside the Apostle of Allah, may Allah bless him and his family. For there is nobody worthier of him than me. When you are prevented from that, then do not shed blood into the cupping-glass in (carrying) it (out)."
Ibn `Abd al-Bir said: "Al-Husayn came to al-Hasan. Thus al Hasan said: `My brother, I have been give poison to drink three times but I have never been given poison like this. I am going to spew my liver.' So al-Husayn said: `Who gave you the poison to drink?' `Why do you ask about that? Do you want to fight against them?" asked al Hasan. "Oppose them before Allah."
Al-Tabari said in Dala'il al-Imama: "The cause of the death of al Hasan was that Mu'awiya poisoned him seventy times. However, the poison did not act upon him. Then he sent (someone) to his (i.e., al Hasan's) wife Ju'da bint Muhammad b. al-Ash'ath b. Qays al-Kindi and gave her twenty thousand dinars, and ten estates from Iraq. Also he ensured her to marry her to his son Yazid. Thus she gave al-Hasan poison to drink. The poison was (mixed with) sweetened flour in a gold bowl." 
Allah, the Great and Almighty said: "However, if you held command, you were sure to make mischief in the land and cut off the ties of kinship!" (Qur'an 47:22)
 Al-Mas'udi (Hamish b. al-Athir, vol. 5, p. 198) and al-Bayhaqi (al Mahasin wa al-Masawi', vol. 1, p. 64) have mentioned: "Al-Hasan, peace be on him, strove (to take) security for Marwan when he was captured at the Battle of the Camel. It was said that Marwan hid in the house of a woman in Basrah."
In Nahj al-Balagha (vol. 1, p. 121), al-Sharif al-Radi said: "When Marwan was taken prisoner at the Battle of the Camel, he asked al-Husayn, peace be on them, to intercede for him with the Commander of the faithful. Thus the latter released him. Then they said: `Commander of the faithful, he (i.e., Marwan) desires to pledge allegiance to you.' Thus he, peace be on him, said: `Did he not pledge allegiance to me after the killing of `Uthman? I am in no need of his pledge of allegiance. For (his hand) is a Jewish hand. If he pledge allegiance to me with his hand, he would violate it after awhile. Well, he is to get power (as short) as the dog licks its nose. He is the father of the four sons. The people will face hard days because of him and his sons.'"
I (i.e., the author) say: Marwan was ungrateful for al-Hasan when he tried to convince Ju'da to poison him. He was as they say: "A tree is known by its fruit."
 Al-Mas'udi, Hamish b. al-Athir, vol. 6, p. 55- 56.
 Ibn `Abd al-Bir al-Maliki, al-Isti'ab.
 Ibn Qutayba al-Dinawari (died 276), al-Imama wa al-Siyasa, pp. 159-
60. Al-Ya'qubi and al-Mas'udi have mentioned words similar to these.
 Ibn Abu al-Hadid, Sharh Nahj al-Balagha, vol. 6, p. 57.
 Al-Ya'qubi, Ta'rikh, vol. 2, p. 200. Al-Mas'udi, Hamish b. al-Athir, vol. 6, p. 57.
 Al-Hakim, al-Mustadrak (Kufa), vol. 6, p. 5.
 Al-Tabari, Dala'il al-Imama, p. 61.