Decades of fighting with the Byzantine Empire (especially the war of 602–628) drained Sasanian finances and manpower. The strain made it harder for the state to defend its frontiers or keep powerful nobles and generals under control. This weakened backdrop shaped how quickly the later Arab-Muslim armies advanced.
After the end of the long Byzantine war, the Sasanian Empire went through a violent succession crisis and factional conflict. Rival leaders and armies competed for power, undermining centralized authority in Ctesiphon. This instability reduced the empire’s ability to respond as a unified state when new invasions began.
Yazdegerd III took the throne as a child and never fully controlled the empire’s competing elites. His reign began during a period of deep internal weakness, and the Arab invasion began soon afterward. He would spend much of his reign fleeing from one region to another trying to organize resistance.
Under the early caliphs, Muslim armies began a sustained campaign into Sasanian-controlled Mesopotamia. These operations targeted frontier towns and routes leading toward the imperial heartland around Ctesiphon. The fighting in Iraq created the staging ground for later decisive battles against the Sasanian field army.
At Walaja in Mesopotamia, forces led by Khalid ibn al-Walid defeated a Sasanian army and its allies. The victory helped the invaders consolidate momentum in the Iraqi theater during the opening phase of the conquest. It also showed that Sasanian forces could be beaten repeatedly before a decisive set-piece battle occurred.
At al-Qadisiyyah (often dated 636, with some sources placing it in 637), the Rashidun army defeated a major Sasanian force. The outcome killed or removed key Sasanian leadership and opened the way to the imperial capital. Historians note that sources disagree on the exact date and scale of forces, but agree on the battle’s decisive effect.
After al-Qadisiyyah, Rashidun forces moved on Ctesiphon, the main Sasanian capital complex on the Tigris. The city fell after a siege lasting from roughly January to March 637, and Yazdegerd III fled east. Losing Ctesiphon meant the Sasanians no longer controlled their political center or the wealth of lower Mesopotamia.
Soon after the fall of Ctesiphon, a Sasanian force attempted to block further advances at Jalula (Jalawla) in Iraq. The Rashidun victory removed another major organized obstacle west of the Zagros Mountains. This helped shift the war from Mesopotamia into the Iranian plateau itself.
In the southwest (Khuzestan), Shushtar held out as a major fortified stronghold under the Sasanian commander Hormuzan. After a long siege (641–642), the city fell, reducing the Sasanians’ ability to defend the wealthy lowlands and river routes. This loss also highlighted how defections and local politics could decide sieges, not just battlefield victories.
At Nahavand in 642, Arab and Sasanian forces fought a decisive battle that later tradition called the “Victory of Victories.” Britannica describes the defeat as a major turning point that paved the way for the Arab conquest and the Islamization of Iran. After this loss, Yazdegerd III could not raise another strong imperial army.
Following Nahavand, Yazdegerd III moved from province to province seeking support from local rulers and nobles. With the imperial center already lost and the main field armies destroyed, resistance increasingly became regional and fragmented. This phase marks the transition from an imperial war to the final collapse of Sasanian rule in the east.
In 651, Yazdegerd III was killed near Merv (Marw), ending the Sasanian dynasty. Britannica treats his death as the point by which the Arab conquest of Iran was completed. With the last shah gone, the Sasanian Empire no longer existed as a political state, even though Iranian culture and local elites continued under new Islamic rule.
Islamic Conquest and Fall of the Sasanian Empire (633–651 CE)