After the death of Prophet Muhammad in 632, the Rashidun Caliphate began a period of unprecedented expansion. The two established world powers, the Byzantine and Sasanian Empires, had just finished a long and exhausting war. This exhaustion, combined with internal divisions, made them vulnerable to the highly motivated and mobile Rashidun armies.
In the West, Islamic forces moved across Syria and Palestine. A major victory at the Battle of Yarmouk in 636 forced the Byzantines to retreat to Anatolia, leaving the Levant to the Caliphate. Jerusalem surrendered in 637. Soon after, General Amr ibn al-Aas invaded Egypt, capturing the strategic Babylon Fortress and the capital, Alexandria, by 642.
In the East, the campaign against the Sasanian Empire proved equally dramatic. Following initial raids, a massive confrontation took place at the Battle of Qadisiyah in 636. Despite the Sasanians' use of war elephants and heavy armor, the Rashidun forces achieved a stunning victory. This led to the capture of the Sasanian capital, Ctesiphon, the following year.
The final blow to the Sasanian central power came at the Battle of Nahavand in 642, often called the 'Victory of Victories.' While the empire officially ended in 651 with the death of Yazdegerd III, the conquest of the vast Persian plateau was a gradual process. Some regions, like the mountainous Tabaristan, remained independent for over a century, while others required persistent military campaigns to fully integrate into the Caliphate.
These conquests weren't just about territory; they brought a new religion, language, and administrative system to the Middle East. The Caliphate's ability to govern diverse populations through early treaties like the 'Covenant of Umar' helped stabilize the vast new empire.