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In page Umar:

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Umar issued an order to invade the very homeland of the Christian Arab forces besieging Emesa, the Jazira. A three-pronged attack against Jazira was launched from Iraq. To further pressure the Christian Arab armies, Umar instructed Sa'd 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.[citation needed]