<s>[INST] Why did the Sasanian empire fall? [/INST] The Sasanian Empire lasted until 651 CE when it was overthrown by the Arab Rashidun Caliphate.</s>\ <s>[INST] How it was overthrown? [/INST] The Sasanian Empire was overthrown by Arab-Muslim armies during the Islamic conquest of Iran in the 7th century. The Arab-Muslim forces, led by Caliph Umar, won a decisive victory against the Sasanian army at the Battle of al-Qādisiyyah in 637 CE, which marked the beginning of the fall of the Sasanian Empire. Over the next several years, the Arab-Muslim armies continued to conquer territories controlled by the Sasanians, ultimately leading to the collapse of the empire.</s>