The Battle of al-Qadisiyyah took place between the armies of the Rashidun Caliphate and the Sasanian Empire in November 636. It resulted in a decisive victory for the Rashidun army and is considered to be one of the most significant engagements of the Muslim conquest of Persia, and thereby of the early Muslim conquests as a whole. After losing al-Qadisiyyah, the Sasanian army was left unable to defend against the Rashidun siege of Ctesiphon, thus being forced to retreat from all of Mesopotamia. This development enabled further Rashidun offensives into the Persian mainland and culminated in the Sasanian Empire's annexation by 651.