<s>[INST] Why did the Sasanian empire fall? [/INST] The Sasanian Empire fell due to a combination of internal decline and external pressure. Internal factors such as weak leadership, economic decline, and political instability weakened the empire, making it vulnerable to invasion. External pressure came in the form of the Arab-Muslim conquest of Iran in the 7th century. The Arab-Muslim armies, led by Caliph Umar and General Khalid ibn al-Walid, defeated the Sasanian forces and brought an end to the empire.</s>\ <s>[INST] Tell me more about Arab-Muslim conquest of Iran [/INST] 