Why were the Persians so easily conquered by the new Islamic empire?

Answer 1

There were four primary reasons that the Sassanian Persian Empire crumbled in the face of Arab Muslim conquerors.

1) General Strength of Nomadic Soldiers: Historically, prior to 19th century weaponry, an enemy who was mounted and able to move quickly could encircle and achieve tremendous successes over settled empires. This situation replayed itself quite often and was a worldwide phenomenon. Many Chinese Imperial Sources stress how important it was to maintain a string of "Cooked Barbarian allies", who were small tribal nations along the Chinese border, but who had the same horseman skills as the more ruthless and adversarial :"Uncooked" Mongol Tribes. The advantage of being on horseback meant that without these Cooked Barbarians to intercede with the Mongols, the Chinese Empire, with the largest armies in the world at that time, would be ill-equipped to defend its territory.

In the case of the Sassanian Persians and the Arab Muslims, there was much the same issue. While the Persians had a large military, it could not compete with the speed and dexterity of the Arab Muslims who had a quick cavalry and had experience as raiding parties and in wars with both sedentary and nomadic enemies. To further push the balance towards the Arabs, King Yazdagerd III made the fatal flaw of trying to fight the Arabs on open plains where horses are most advantageous rather than wait for the Arabs to try to besiege major Persian cities, where the Arab lack of siege equipment would weaken their position.

2) Byzantine-Sassanian Wars: The Byzantines and Sassanians considered each other the only real civilizations in Middle East less than a few decades before the Rise of Islam and fought each other numerous times over control of what is today East Turkey and Iraq. These were intense religious wars (Christian vs. Zoroastrian) and greatly weakened both empires at just the wrong time. When the Arabs faced the Persians, the size of the Sassanian Persian Army was less than half of what it had been prior to fighting with the Byzantines. Additionally, since Arabia had been seen as a cultural and political backwater barely worth notice prior to the Rise of Islam, the Sassanians maintained a porous southern defense and miscalculation of the strength, experience, and numbers of the invading Islamic Armies.

3) Cultural Traits of the Arabs: Arab Tribes and warriors owed their loyalty to their Sheikhs and leaders without fault because the Sheikh provided each person with all of the amenities and protection necessary to live a full life. Whenever the Arabs went to war, the Sheikhs could count on the ability to muster their tribesmen as soldiers without issue. This meant that relatively large numbers of men could be called upon to join Sa3d ibn Abi Waqqas (one of the Islamic Empire's chief generals) in his invasion of Persia.

4) Persian Muslims and Allegiance of Persian Nobles: Among the Muslims coming out of Arabia were a number of Persian slaves that had been sold in Arabia and freed by Muhammad, such as his Apostle Salman the Persian. These Persian Muslims understood the lay of the land well and after the Fall of Ctesiphon, the Sassanian capital city, were able to serve as effective administrators until Persia could effectively be added to the Caliphate. Additionally, Persian Muslims like Salman recognized that the Persian nobility, if given a position to retain their power, could be counted on to support the new Islamic administration, since these nobles would rather serve a different master than plunge Persia into a decades-long civil war. As a result, by simply converting to Islam, Persian nobles could retain their holdings and even acquire the holdings of nobles who failed to convert to Islam or declare loyalty to the Caliph.