After Imam Ali, Imam Hasan attained to the caliphate. At this time the Muslim society had assumed a peculiar form. Their strength had been divided almost evenly, and if Imam Hasan had continued fighting against Mu'awiya neither of the two parties could expect victory without severe bloodshed. The result of such a fighting would have been disastrous for the Muslims. Hence, Imam Hasan was faced with a situation in which he had no alternative but to come to terms with Mu'awiya and prevent senseless bloodshed. If he had persisted in fighting it would have resulted in gain to the Eastern Roman Empire on the external front and to the Khawarij within the Islamic territories. If those 400,000 or 500,000 Muslims had attacked one another on that day and fighting with Mu'awiya had been continued only Allah knows what calamity would have fallen on the Muslims from the side of the Eastern Roman Empire and what dimensions the danger from Khawarij would have assumed, and what the history of Islam would have been!
Hence, Imam Hasan retired from the caliphate and thus safeguarded the blood of the Muslims and the strength of Islam, and prevented the external and internal enemies from taking undue advantage of the situation. This does not, however, mean that he surrendered to Mu'awiya and recognized him as the caliph and the Commander of the Faithful.
One of the conditions of the Peace Treaty between Imam Hasan and Mu'awiya reads thus: "Hasan bin Ali makes Peace on the condition that he will not be under an obligation to call Mu'awiya, the Commander of the Faithful". It meant that he did not recognize Mu'awiya to be the caliph and the Commander of the Faithful. Those, who think that by retiring, Imam Hasan bin Ali surrendered to Mu'awiya and Mu'awiya became the caliph of the Muslims and Hasan bin Ali also became one of his obedient subjects, should keep in mind this valuable narration by Ibn Athir which refutes this false belief:
"After Hasan bin Ali retired and Mu'awiya became the caliph, Farwa bin Nawfal Ashjaie Kharij, who had deserted the Khawarij earlier along with five hundred men and gone to the city of Zur said: "Now there is no doubt about the fact that we should fight against Mu'awiya's administration. As Mu'awiya has come at the helm of affairs and become the caliph we must wage war against him". They, therefore, marched towards Iraq and reached the palm-grove of Kufa. In the meantime Imam Hasan had left Kufa and was on his way to Madina. When Mu'awiya came to know that the said Khariji had revolted along with his five hundred men, he wrote a letter to Imam Hasan, possibly with a view to strengthening the Peace Treaty with the Imam. He wrote thus: "I understand that Farwa bin Nawfal Khariji is proceeding to Kufa along with five hundred men. I, therefore, direct you to go and fight against him and ward him off, and after you have vanquished him there will be no objection to your proceeding to Madina".
Imam Hasan received Mu'awiya's letter when he had arrived at Qadisiya. He sent him the following reply: "O Mu'awiya! You have appointed Hasan bin Ali to go like one of your officers and to ward off a rebellious Khariji. I, Hasan bin Ali, have retired, in the interest of the Muslims, from the caliphate, which is my right. If I had wished to fight against one of the people of the Qibla i.e. with a Muslim, whoever he might be, and to which ever sect he might belong, I would have fought against you in the first instance".
The Imam meant to say that he desisted from fighting against Mu'awiya in spite of the latter's being deviated from Islam as compared with all others. It will be observed that the Imam did not say: "I have recognized you as the caliph". On the other hand he wrote: "I let you go and did not fight against you". And possibly a better interpretation of the words "I let you go" may be this: 'I have set you at liberty in the field of politics and have myself retired only in the interest of Islam and to avoid the bloodshed of the Muslim i.e. I considered it futile that these two forces of Islam which are evenly balanced should fight and kill and weaken each other and be annihilated, and the external and internal enemies should take undue advantage of this situation '.
After the martyrdom of Imam Hasan, Imam Husayn did not also rise up in arms against Mu'awiya during the last ten years of his rule (49 -60 AH) and did not undertake that campaign against him -the campaign, which he considered necessary during the regime of Yazid. However, he continuously criticized and reprimanded Mu'awiya and denied his rightfulness to the caliphate in the same manner in which his brother Imam Hasan had done.
After the martyrdom of his brother Imam Hasan, Imam Husayn wrote the following letter to Mu'awiya that has been reproduced by Ibn Qutayba Denuri:
"O Mu'awiya! Are you not the same person who killed Hujr bin Ady and his pious friends unlawfully? They were the persons, who condemned heresy and ordered the people to do good and restrained them from evil. You killed them cruelly after granting them security and making firm promises and covenants with them. By doing so you defied Allah and considered the Divine covenants to be frivolous. Did you not kill Amr bin Humuq Khuzaie, who was one of the distinguished companions of the Holy Prophet? He was the man whose face had worn out and whose body had grown thin on account of excessive worship. You killed him after granting him security and holding out promises of safety to him. If such promises had been held out to the desert deer, they would have come down to you from the hills with perfect confidence."
Are you not the man who associated Ziyad, the son of an unknown father, with your father Abu Sufyan and called him your brother, as Ziyad bin Abu Sufyan, and supposed that he is the son of Abu Sufyan although the Holy Prophet has said: "The child belongs to the man on whose bed (i.e. in whose house) he is born and the woman giving birth to the child is married to him and the adulterer is to be stoned to death as commanded by Allah. And then you have appointed Ziyad to rule over the Muslims so that he may kill them and amputate their hands and feet and hang them on date-palm trees. Allah be praised!
"O Mu'awiya! It appears that you are not a member of the Muslim community and the Muslims have no connections with you. Fear Allah and beware of the Day of Judgment, because Allah has a document from which nothing, whether small or big or good or bad, is omitted and everything is taken into account. You must remember that Allah does not forget these acts of yours, that is, you kill people on mere suspicions and false accusations and have made a boy, the ruler of the Muslims, who drinks wine and plays with dogs.
"O Mu'awiya! I see that you have destroyed yourself, spoiled your faith and made the Muslim ummah helpless".
This was the manner in which Imam Hasan and Imam Husayn, the two sons of the Holy Prophet addressed and wrote letters to Mu'awiya bin Abu Sufyan and called him to account.