In the spring of 632, a grandson of Khosrau I, Yazdegerd III who had lived in hiding, ascended the throne. In that same year, the first raiders from the Arab tribes made their raids into Persian territory. They were united by Islam. Years of warfare had exhausted both the Byzantines and the Persians. The Sassanids were further weakened by economic decline, heavy taxation, religious unrest, rigid social stratification, the increasing power of the provincial landholders, and a rapid turnover of rulers. These factors made the Islamic conquest of Persia easier than it would have been in earlier times.