In 810, the caliph al-Amin, and his brother, Al-Ma'mun, came in conflict which each others, which later led to a civil war; in January 811, al-Amin formally began the Great Abbasid Civil War when he appointed Ali ibn Isa as governor of Khurasan, placed him at the head of an unusually large army of 40,000, drawn from an elite group known as "abna′", and sent him to depose al-Ma'mun. When Ali ibn Isa set out for Khurasan, he reportedly took along a set of silver chains with which to bind al-Ma'mun and carry him back to Baghdad. The news of Ali's approach threw Khurasan into panic, and even al-Ma'mun considered fleeing. The only military force available to him was a small army of some 4,000–5,000 men, under Tahir. Tahir was sent to confront Ali's advance, but it was widely regarded as almost a suicide mission, even by Tahir's own father. The two armies met at Rayy, on the western borders of Khurasan, and the ensuing battle (3 July 811) resulted in a crushing victory for the Khurasanis, in which Ali was killed and his army disintegrated on its flight west.