In the early 600s, the Sasanian Empire was strained by a long, costly war with the Byzantine Empire. This left the state financially and militarily weakened just before Arab Muslim armies began major expansion beyond Arabia.
Yazdegerd III came to the throne as the last ruler of the Sasanian dynasty. His reign began while the empire was already unstable, and he never gained full control over powerful nobles and generals.
Rashidun forces under Khalid ibn al-Walid began a major push into Sasanian Mesopotamia (roughly modern Iraq). This opened the wider war by targeting the Sasanian Empire’s political and economic center.
In May 633, Rashidun forces defeated Sasanian and allied troops at Walaja in Mesopotamia. The victory helped the invaders keep momentum during the first year of the campaign in Iraq.
In 633, the Battle of Ullais ended in a Rashidun victory over Sasanian forces in Mesopotamia. This further weakened local Sasanian defenses and helped the invading army move against key towns and routes.
Around January 634, fighting at Firaz took place near the edge of Sasanian-held territory. Even though details are debated, the episode reflects how quickly the conflict spread to border areas and rival imperial interests.
In mid-November 636, Rashidun forces defeated a major Sasanian army at al-Qadisiyyah. The result opened the way toward the Sasanian capital region and became a decisive turning point in the war.
From January to March 637, Rashidun armies besieged and captured Ctesiphon, the Sasanian capital. With the capital occupied, Sasanian leadership and resources were forced eastward, and Muslim rule consolidated across much of Mesopotamia.
In April 637, a Rashidun victory at Jalula helped secure territory west of the Zagros Mountains. This mattered because it reduced the chance of a quick Sasanian counteroffensive to retake Iraq and the former capital region.
In 642, Arab and Sasanian forces fought at Nahavand in western Iran. The Sasanian defeat was a major turning point that paved the way for the conquest of the Iranian plateau and the broader Islamization of Iran over time.
In November 644, Caliph Umar ibn al-Khattab was assassinated, and leadership passed soon after to a new caliph. The conquests continued, but the war now depended more on provincial commanders and shifting local loyalties across former Sasanian lands.
By 650–651, fighting in Fars (a core region of southwest Iran) included the Battle of Istakhr. These campaigns helped end large-scale Sasanian control in the Iranian heartland and pushed the last king farther east.
After years of retreat and failed attempts to rebuild an army, Yazdegerd III was killed at Merv in 651. His death removed the central symbol of Sasanian authority and made coordinated resistance much harder.
By 651, after Nahavand and the death of Yazdegerd III, Arab Muslim forces had effectively completed the conquest of the former Sasanian Empire. The political map changed: Sasanian institutions ended, and Iran was drawn into the expanding caliphate, setting long-term cultural and religious transformation in motion.
Arab conquest of the Sasanian Empire (633–654)