After the Battle of Nahavand, Omar launched a full-scale invasion of the Sassanid Persian Empire. The invasion was a series of well-coordinated multi-pronged attacks designed to isolate and destroy their targets. Omar launched the invasion by attacking the very heart of Persia, aiming to isolate Azerbaijan and eastern Persia. This was immediately followed by simultaneous attacks on Azerbaijan and Fars. Next, Sistan and Kirman were captured, thus isolating the stronghold of Persia, the Khurasan. The final expedition was launched against Khurasan, where, after the Battle of Oxus River, the Persian empire ceased to exist, and Yazdegerd III fled to Central Asia.