Imam Hassan Mujtaba - upon whom be peace - was the second Imam. He and his brother Imam Husayn were the two sons of Amir al-mu’minin Ali and Hadrat Fatimah, the daughter of the Prophet. Many times the Prophet had said, "Hassan and Husayn are my children." Because of these same words Ali would say to his other children, "You are my children and Hassan and Husayn are the children of the Prophet."
Imam Hassan was born in the year 3 A.H. in Medina and shared in the life of the Prophet for somewhat over seven years, growing up during that time under his loving care. After the death of the Prophet which Imam Hassan was placed directly under the care of his noble father. After the death of his father, through Divine Command and according to the will of his father, Imam Hassan became Imam; he also occupied the outward function of caliph for about six months, during which time he administered the affairs of the Muslims. During that time Mu’awiayh, who was a bitter enemy of Ali and his family and had fought for years with the ambition of capturing the caliphate, first on the pretext of avenging the death of the third caliph and finally with an open claim to the caliphate, marched his army into Iraq, the seat of Imam Hassan’s caliphate. War ensued during which Mu’awiyah gradually subverted the generals and commanders of Imam Hassan’s army with large sums of money and deceiving promises until the army rebelled against Imam Hassan.
Finally, the Imam was forced to make peace and to yield the caliphate to Mu’awiyah, provided it would again return to Imam Hassan after Mu’awiyah’s death and the Imam’s household and partisans would be protected in every way.
In this way Mu’awiyah captured the Islamic caliphate and entered Iraq. In a public speech he officially made null and void all the peace conditions and in every way possible placed the severest pressure upon the members of the Household of the Prophet and the Shi’ah. During all the ten years of his imamate, Imam Hassan lived in conditions of extreme hardship and under persecution, with no security even in his own house.
In the year 50 A.H. he was poisoned and martyred by one of his own household who, as has been accounted by historians, had been motivated by Mu’awiyah.
In human perfection Imam Hassan was reminiscent of his father and a perfect example of his noble grandfather. In fact, as long as the Prophet was alive, he and his brother were always in the company of the Prophet who even sometimes would carry them on his shoulders. Both Sunni and Shi’ite sources have transmitted this saying of the Holy Prophet concerning Hasan and Husayn: "These two children of mine are Imams whether they stand up or sit down" (allusion to whether they occupy the external function of caliphate or not).
Source: SHI’ITE ISLAM by Allamah Sayyed Muhammad Husayn Tabatabai