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Ali was assassinated in 661 by a Kharijite partisan. Six months later in the same year, in the interest of peace, Hasan ibn Ali, highly regarded for his wisdom and as a peacemaker, and the Second Imam for the Shias, and the grandson of Muhammad, made a peace treaty with Muawiyah I. In the Hasan-Muawiya treaty, Hasan ibn Ali handed over power to Muawiya on the condition that he be just to the people and keep them safe and secure, and after his death he not establish a dynasty. This brought to an end the era of the Rightly Guided Caliphs for the Sunnis, and Hasan ibn Ali was also the last Imam for the Shias to be a Caliph. Following this, Mu'awiyah broke the conditions of the agreement and began the Umayyad dynasty, with its capital in Damascus.

Who killed Ali?
Kharijite partisan