Fall of the Sasanian Empire

The Sasanian era is one of the most influential periods in Iran's history. It also marks the third rise of a great Iranian empire, a dynasty that rivaled its predecessor, the Achaemenids who too, like the Sassanids were native to the province of Pars, and in some instances the Parthians, in glory and power. Although it was at war with the Roman Empire, its neighboring archrival for over 400 years, and the two fought many wars during the over 700-years lasting Roman-Persian Wars, the Sasanian Empire met its demise not by the Byzantine-Roman Empire, but by emerging Arab Muslims from its southern borders. However, the conflict with the Byzantines greatly contributed to its weakness, by draining Sassanid resources, leaving it a prime target for the Muslims.

Sassanid 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. Rapid turnover of rulers and increasing provincial landholder power further diminished the Sassanids. 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.

Years of warfare between the Sasanids and the Byzantines, as well as the strain of the Khazar invasion of Transcaucasia, had exhausted the army. No effective ruler followed Khosrau II and this created chaos in society and problems in the provincial administration (until Yazdegerd III came to power). All these factors undermined the strength of the Persian army. Yazdegerd III was merely 8 years old when he came to the throne and lacked experience and didn't try to rebuild the army. The Sasanid Empire was highly decentralized, and was in fact a "confederation" with the Parthians, who themselves retained a high level of independence. However, after the last Sasanid-Byzantine war the Parthians wanted to withdrew from the confederation, and the Sasanians were thus ill-prepared and ill-equipped to mount an effective and cohesive defense against the Muslim armies. Moreover, the powerful northern and eastern Parthian families, the Kust-i Khwarasan and Kust-i Adurbadagan, withdrew to their respective strongholds and made peace with the Arabs, refusing to fight alongside the Sassanids.

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Wikipedia