 Abu Bakr asserted his authority in a successful military campaign known as the Ridda wars, whose momentum was carried into the lands of the Byzantine and Sasanian empires.By the end of the reign of the second caliph, Umar I, Arab armies, whose battle-hardened ranks were now swelled by the defeated rebels and former imperial auxiliary troops, conquered the Byzantine provinces of Syria and Egypt, while the Sassanids lost their western territories, with the rest to follow soon afterwards.Umar improved administration of the fledgling empire, ordering improvement of irrigation networks and playing a role in foundation of cities like Basra.