Imam Reza's (عليه السلام) acceptance of the caliphate from Al-Ma'mun is indeed a historical event with multifaceted reasons, often cited in authentic hadith. Here is a brief exploration of some of the reasons behind his succession
The historic event of Karbala in 61 AH witnessed the unparalleled sacrifice of Imam Hussain ibn Ali (عليه السلام), the grandson of Prophet Muhammad (صلى الله عليه وسلم).