they are like everyone else: command is only given to the most competent, and, once the battle is over, the commander returns to his previous position. On the eve of the campaign, Umar, in order to boost morale, decided to reinstall Khalid as field commander, four years after his dismissal. Khalid's reputation as the conqueror of the Eastern Roman provinces demoralized the Persian commanders, most of whom had already been defeated by him during his conquest of Mesopotamia in 633. Before Umar could issue the reappointment order, Khalid died in Emesa. Throughout the Persian campaign, Umar even appointed the commanders of the wings, the center and the cavalry of the army. Umar strictly instructed his commanders to consult him before making any decisive move in Persia. All the commanders, before starting their assigned campaigns, were instructed to send a detailed report of the geography and terrain of the region and the positions of the Persian garrisons, forts, cities and troops. Umar then would send them a detailed plan of how he wanted the region to be captured. Only the tactical issues were left to the field commanders to tackle in accordance with the situation they faced at their fronts. Umar appointed the best available and well-reputed commanders for the campaign. Conquest of Central Iran In the wake of Khalid's demise, Umar appointed Abdullah ibn Uthman as commander of the Muslim forces for the invasion of Isfahan. From Nahavand, Nu'man ibn Muqaarin marched to Hamadan, and then proceeded 370 kilometres (230 mi) southeast to the city of Isfahan, defeating a Sasanian army there. The enemy commander, Shahrvaraz Jadhuyih, along with another Sasanian general, was killed during the battle. Nu'man, reinforced by fresh troops from Busra and Kufa under the command of Abu Musa Ashaari and Ahnaf ibn Qais, then besieged the city. The siege continued for a few months before the city surrendered. In 651, Nu'aym ibn Muqaarin, Nu'man's brother, marched northeast to Rey, Iran, about 320 kilometres (200 mi) from Hamadan, and laid siege to the city, which surrendered after fierce resistance. Nu'aym then marched 240 kilometres (150 mi) northeast toward Qom, which was captured without much resistance. This represented the boundary of the Isfahan region. Further northeast was Khurasan, and southeast lay Sistan. Meanwhile, Hamadan and Rey had rebelled. Umar sent Nu'aym, whose brother Nu'man had recently died, to Hamadan to crush the rebellion and clear Isfahan's western frontier.