The Siege of Herat is part of the Islamic conquest of Persia.

Caliph Umar (634-644 CE) launches an offensive against the Sassanid Persian Empire in 642 and by late 651 almost all of the Sassanid empire is conquered, except Khorasan.

A veteran military commander, Ahnaf ibn Qais, is appointed by the caliph for the conquest of Khurasan, which in these time comprises most of present-day north eastern Iran, Afghanistan and Turkmenistan.

By 651, Ahnaf has captured Khorasan and the last Sassanid emperor Yazdegerd III is defeated in his final struggle against the Caliphate at the Battle of Oxus river, whereupon he flees to Merv and eventually is murdered by a miller in 651.

With him ends the last non-Muslim Iranian dynasty.