Why did the Sasanian empire fall?
The direct cause of downfall of the Sasanian empire, also known as the Sassanid empire, was the Arab conquest which took place between 637 and 651 AD. The Rashidun Caliphate, the most powerful geopolitical force in Asia and the Middle East at that time, invaded Asōristān, a province of the Sasanian empire roughly overlapping with modern day Iraq. This province was the economic and cultural center of the Sasanian empire, and the Sasanians eventually forced the Rashidun army to give up their holdings until a second attack which got much farther in 636. For the next six years, the natural barrier created by the Zagros mountains stonewalled the attacks by the Arabs; however, by 642 the Rashidun Caliphate brought a full scale invasion of Persia (including the Sasanian empire.) By 651 AD, they had taken complete control over the Sasanian empire.
How did that develop afterward? Briefly summarize the history of the next 100 years.