Battle of Siffin
In 661 CE, after Ali's assassination, Hasan ibn Ali attained to the caliphate. There was a military conflict between Caliph Ali and Muawiya ibn Abu Sufyan (see Battle of Siffin); But upon the insistence of Muawiya, Hasan signed the Hasan–Muawiya treaty with Muawiyah. According to the treaty, Hasan ceded the caliphate to Muawiyah but the latter could name no successor during his reign; instead, he was to let the Islamic world choose its successor afterward.