<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 the Arab-Islamic Conquest in the 7th century. The empire was defeated by a combination of Arab military forces and internal rebellion, which weakened the Sasanian hold on its territories. The final defeat of the Sasanian army at the Battle of al-Qādisiyyah in 637 marked the end of the empire, and the Arab-Islamic forces went on to establish the Islamic Caliphate in its place.</s>