Through the Peace Treaty...
Through the Peace Treaty, al-Hasan handed authority to Mu'awiya provided that the latter should act according to the Book of Allah, the Sunna (practices) of the Prophet, may Allah bless him and his family, and the Sira (practices) of the Orthodox Caliphs. In this way, al-Hasan was able to reduce the authority of his enemy. In other words he reduced the violations of his enemy through the foregoing conditions.
That is because the violations of Mu'awiya cannot be counted and were severe in punishment. This indicates that al-Hasan was the most knowledgeable of all people in Mu'awiya and his moral abilities towards these conditions. Moreover, the Peace Treaty was the document which the two parties (i.e., al-Hasan and Mu'awiya) signed to conform to the conditions where in.
Accordingly, the Peace Treaty was at most the matter of limited material things which one of the two parties (i.e., Mu'awiya) wanted to get. In the meantime it was the matter of limitless spiritual things which the second party (i.e., al-Hasan) obtained. Through his reconciliation with al-Hasan, peace be on him, Mu'awiya did not intend to get anything but to take the reins of government.
However, when al-Hasan handed over authority to Mu'awiya, he intended to safeguard his doctrine from distortion and his Shi'a from destruction, and to underline that the usurped right (to authority) should be returned on the day when Mu'awiya would die. It is an act of correctness that we do not understand the meaning of the Peace Treaty but through this facet. To understand clearly the aims of the two parties, we have to analyze their words on the day when they made peace with each other. 1.
The Declarations of the two Parties It is enough for us to mention the words which Mu'awiya said concerning the Peace Treaty with al-Hasan, peace be on him. Many historians have narrated these words on his authority. Among them is b. Kathir, who has reported these words of Mu'awiya: "We are satisfied with it (i.e., the Peace Treaty) (to take the reins of) government.
[1] When Mu'awiya exchanged letters with al-Hasan to pave the way to the Peace Treaty, he said: "It is for you that you are not ruled with wrong, the affairs are not executed without you, and you are not disobeyed in any affair." [2] It is enough for us to mention the declarations which al-Hasan said to make his Shi'a understand the backgrounds of his reconciliation with Mu'awiya: "You do not know what I have done.