change in timeline means that the Arab conquest started precisely when the Sassanians and Parthians were engaged in internecine warfare over succession to the Sassanian throne. Sasanian Empire before the conquest Since the 1st century BC, the border between the Roman (later Byzantine) and Parthian (later Sasanian) empires had been the Euphrates River. The border was constantly contested. Most battles, and thus most fortifications, were concentrated in the hilly regions of the north, as the vast Arabian or Syrian Desert (Roman Arabia) separated the rival empires in the south. The only dangers expected from the south were occasional raids by nomadic Arab tribesmen. Both empires therefore allied themselves with small, semi-independent Arab principalities, which served as buffer states and protected Byzantium and Persia from Bedouin attacks. The Byzantine clients were the Ghassanids; the Persian clients were the Lakhmids. The Ghassanids and Lakhmids feuded constantly, which kept them occupied, but that did not greatly affect the Byzantines or the Persians. In the 6th and 7th centuries, various factors destroyed the balance of power that had held for so many centuries. The conflict with the Byzantines greatly contributed to its weakness, by draining Sassanid resources, leaving it a prime target for the Muslims. Social problems Sasanian society was divided into four classes: priests, warriors, secretaries, and commoners. The latter formed the bulk of the population, served as its sole tax base, and remained its poorest class. At the climax of Khosrau II's ambitious Byzantine territory conquests in the Levant and much of Asia Minor, taxes rose dramatically, and most people could not pay. Years of Sassanid-Byzantine wars had ruined trade routes and industry, the population's main income sources. The existing Sassanid administrative structure proved inadequate when faced with the combined demands of a suddenly expanded empire, economy, and population. Rapid turnover of rulers and increasing provincial landholder (dehqan) power further diminished the Sasanians. Over a period of fourteen years and twelve successive kings, the Sassanid Empire weakened considerably, and the power of the central authority passed into the hands of its generals. Even when a strong king emerged following a series of coups, the Sassanids never completely recovered. Events Revolt of the Arab client states (602) The Byzantine clients, the Arab Ghassanids, converted to the Monophysite form of Christianity, which was regarded as heretical by the established Byzantine Eastern Orthodox Church. The Byzantines attempted to suppress the heresy, alienating the Ghassanids and