in july, 'amr wrote to 'umar requesting reinforcement; but before the letter reached him, the caliph had already dispatched the first reinforcement, which was 4,000 strong. the army was composed mostly of the veterans of the syrian campaigns. even with these reinforcements, 'amr was unsuccessful. by august 640, 'umar had assembled another 4,000 strong force, which consisted of four columns, each of 1,000 elite men. zubair ibn al-awam, a renowned warrior and commander, veteran of the battle of yarmouk and once a part of khalid ibn walid's elite mobile guard, was appointed the supreme commander of army—'umar had indeed offered zubair the chief command and governorship of egypt, but zubair had declined.