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Ubayd Allah ibn al-Abbas was the vanguard leader of Hasan ibn Ali, the son of Ali ibn Abi Talib and the fourth caliph of the Rashidun Caliphate. Hasan's vanguard played a crucial role in the Battle of Siffin, a pivotal conflict between Ali and Mu'awiya ibn Abi Sufyan, the founder of the Umayyad Caliphate.