Umar
Military legacy

Along with Khalid ibn Walid, Umar was influential in the Ridda wars.

One strategic success was his sundering of the Byzantine-Sassanid alliance in 636, when Emperor Heraclius and Emperor Yazdegerd III allied against their common enemy. He was lucky in that the Persian Emperor Yazdegerd III couldn't synchronize with Heraclius as planned. Umar fully availed himself of the opportunity by inducing the Byzantines to act prematurely. This was contrary to the orders of Emperor Heraclius, who presumably wanted a coordinated attack along with the Persians. Umar did this by sending reinforcements to the Roman front in the Battle of Yarmouk, with instructions that they should appear in the form of small bands, one after the other, giving the impression of a continuous stream of reinforcements that finally lured the Byzantines to an untimely battle. On the other hand, Yazdegerd III was engaged in negotiations that further gave Umar time to transfer his troops from Syria to Iraq. These troops proved decisive in the Battle of Qadisiyyah.

His strategy resulted in a Muslim victory at the Second Battle of Emesa in 638, where the pro-Byzantine Christian Arabs of Jazira, aided by the Byzantine Emperor, made an unexpected flanking movement and laid siege to Emesa (Homs).

Umar issued an order to invade the very homeland of the Christian Arab forces besieging Emesa, the Jazirah. A three-pronged attack against Jazirah was launched from Iraq. To further pressure the Christian Arab armies, Umar instructed Saad ibn Abi Waqqas, commander of Muslim forces in Iraq, to send reinforcements to Emesa. Umar himself led reinforcements there from Medina. Under this unprecedented pressure, the Christian Arabs retreated from Emesa before Muslim reinforcements could arrive. The Muslims annexed Mesopotamia and parts of Byzantine Armenia.

After the Battle of Nahavand, Umar 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. Umar 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.
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