campaign from Medina, appointing an astute field commander for Nahavand. Umar acquiesced, appointing Mugheera ibn Shuba as commander of the forces concentrated at Medina, and Nouman ibn Muqarrin as commander-in-chief at Nahavand. The Muslim army first concentrated at Tazar, and then defeated the Persians at the Battle of Nahavand in December 642. Nouman died in the action, and, as per Umar's instructions, Hudheifa ibn Al Yaman became the new commander-in-chief. Thereafter, the Muslims captured the whole district of Hamadan, encountering only feeble resistance. Conquest of Persia (642–651) After several years, Caliph Umar adopted a new offensive policy, preparing to launch a full-scale invasion of what remained of the Sasanian Empire. The Battle of Nahavand was one of the most decisive battles in Islamic history and proved to be the key to Persia. After the devastating defeat at Nahavand, the last Sassanid emperor, Yazdegerd III, fled to different parts of Persia to raise a new army, with limited success, while Umar attempted to capture him. Strategic planning for the conquest of Persia Umar decided to strike the Persians immediately after their defeat at Nahavand, while he still possessed a psychological advantage. Umar had to decide which of three provinces to conquer first: Fars in the south, Azerbaijan in the north or Isfahan in the center. Umar chose Isfahan, as it was the heart of the Persian Empire and a conduit for supply and communications among the Sasanian garrisons, and its capture would isolate Fars and Azerbaijan from Khorasan, Yazdegerd's stronghold. After he had taken Fars and Isfahan, the next attacks would be simultaneously launched against Azerbaijan, the northwestern province, and Sistan, the easternmost province of the Persian Empire. The conquest of those provinces would leave Khorasan isolated and vulnerable, the last stage of the conquest of Sassanid Persia. Preparations were complete by January 642. The success of the plan depended upon how effectively Umar could coordinate these attacks from Medina, about 1500 kilometers from Persia, and upon the skill of his field commanders. Umar adopted a different approach to the command structure. Instead of appointing a single field commander to press the campaign, Umar appointed several commanders, each assigned a different mission. Once a commander's mission ended, he would become an ordinary soldier under the new field commander for the latter's mission. The purpose of this strategy was to allow commanders to mix with their soldiers and to remind them that