Well , Sassanid has centralized gov , but there where 7 great family with strong influence .... Ultimately , khsoru II lost in the last battle against Byzantine because internal power struggle and gteat plague and beterayel of his best general . after assaination of khosro II , Sassanid great families start serious civil war ( imagine song of ice and fire / game of thrones situation ) .... In the middle of civil war , Muslims rised and start to attack both Byzance and Sassanid's and you know the result ....
From someone else ( I don't agree with all of it )
The Sassanid Empire was formed by an alliance between the Persian House of Sassan and the Parthian noble Houses, the latter being great feudal families who owned many large fiefdoms in the empire and provided the majority of military manpower for the Sassanid military, especially the heavy cavalry which was the backbone of the army. It was a confederacy between Persians and Parthians whereby the Sassanids were considered by the Parthian nobility to be "first amongst equals". This type of feudal regime had pros and cons of course. On the one hand, it greatly reduced the power of the Shah who was held accountable for his actions. On the other, it allowed the nobility to have great influence on the crown and the affairs of state in general. This socio-political reality was a fertile ground for corruption, court intrigue, and tyranny, because it was formed in the first place by two powerful competing parties who vied for power, wealth, and influence: the crown and the nobility. In such situation it was only expected that when a Shah did not dance to the nobility's tunes, he was usually removed. Only powerful monarchs like Khosrow I were able to have a relatively free hand in running the affairs of the empire without managing to have themselves deposed.
The seeds of the disintegration of the socio-political regime of the Sassanid Empire were planted during the reign of the aforementioned monarch. Khosrow I is celebrated mainly for his extensive reforms. According to the new measures he introduced, the Parthian noble families were shuffled around in line with the newly introduced administrative quadripartition of the empire and the rule of the four generals. For example, members of the Parthian House of Karen, whose traditional territories were in the quarter of the west, were assigned asSpahbeds of the quarter of the east, whilst members of the House of Espahbodan, traditionally in the quarter of the east, were given the generalship of the quarter of the west. This was a very unpopular measure, because it clearly meant that one family found its territories under the rule of members from other families, sometimes rival ones. Also, Khosrow's reforms greatly interfered with the economic and military management of the Parthian realms, which had hitherto been mostly private and untouched by central authority. This greatly reduced the Parthians' ability to amass wealth at the expense of the government and put an end to their economic and military mismanagement. The Parthians never forgot what the Sassanid did under Khosrow I, since he disrupted the "natural order of things".
The internal chaos that began during the reign of Khosrow I's successor, Hormozd IV, made matters forever worse for the Sassanid Empire, because it paved the way for almost never ending vendettas between the House of Sassan and the Parthian noble Houses. Hormozd IV was a "tyrant", because he got sick and tired of Parthian nobles breathing down his neck and decided to purge the nobility of troublesome elements. The slaughter was great but not thorough, since Hormozd was overthrown by Bahram Chubin from the Parthian House of Mehran and two Parthian brothers from the House of Espahbodan, Vestahm and Vanduyeh, having murdered many of their brethren. Instead of installing Khosrow II on the throne, Bahram did the unthinkable and assumed kingship, usurping the divine right of the Sassanids to don the imperial crown. He was defeated by an alliance between the aforementioned Parthian brothers along with their Sassanid candidate Khosrow II and the Romans under Maurice. Sometime later, the reigning Khosrow II decided to avenge his father and murder Vanduyeh, because the latter had participated in the coup that had deposed and killed Hormozd IV, Khosrow's father and legitimate predecessor. As a result, Vestahm got pissed and decided to stage a huge rebellion in the north eastern part of the empire, and in the process he declared himself a king, greatly damaging the legitimacy of the Sassanid crown. Vestahm was later defeated and killed, and everything went back to normal for a period of time.
In the "last great war of antiquity" and during the phase when the Persians were on the eve of world conquest, Vanduyeh's grandson, Rostam, started a revolt in the east, some time in 624. He didn't forget Khosrow murder of his grandfather, so he stabbed his sovereign in the back in the midst of that fateful armed conflict between the superpowers of the ancient world. It became even worse. Vanduyeh's son (and Rostam's father), Farrokh Hormozd, began plotting against Khosrow II, apparently in order to avenge his father. As if that wasn't enough, Shahrvaraz of the House of Mehran, Khosrow's conquering general in the west, threw off his allegiance to the Sassanid crown and struck a deal with both Heraclius and Farrokh Hormozd and also the latter's son, Farrokhzad (Rostam's brother). The Armenians were also involved in the conspiracy. And so during arguably one of the most crucial junctures of world history, the Roman-Parthian-Armenian alliance against Khosrow II was formed, and the result was predictable. Shahrvaraz's armies became collaborators with the Romans (part of the agreement between Shahrvaraz and Heraclius was the Persian evacuation of vast conquered Roman territories), and the only faction that resisted the counter-attacking Roman armies of Heraclius was that of general Shahin, theSpahbed of the quarter of the west. But not much could have been done. The deal between the most powerful figures of the Sassanid and Roman Empires was struck, Khosrow's fate was sealed, and the war was decided before it even came to a formal end.
A period of utter chaos followed the deposition of Khosrow II and the Persian defeat in the final war. The alliance that had brought down Khosrow disintegrated, and the Persian Sassanid Empire was divided into 3 factions as a result, each of which had its own candidate to the throne: 1) the "conquest" faction of general Shahrvaraz, 2) the Parthian faction led by the Espahbodan family (Farrokh Hormozd and his sons Farrokhzad and Rostam), and 3) the Persian faction, backed by the Parthian House of Suren.
After Khosrow II fall, first Kavad II was installed in 628. He was a puppet who ruled for few months before he died because of unknown reasons, most likely a plague. Ardashir III (628-630) came after him. He was a child, and since he was installed on the throne by the other factions without consulting Shahrvaraz, the child was murdered by the rebellious general who afterwards assumed kingship for very short period of time before getting assassinated by the Parthian faction. Borandokht (631) was crowned as empress with the blessing of the Parthian faction. Because she was not favoured by the Persian faction, she was deposed, and her sister Azarmidokht (632) was crowned. Farrokh Hormozd asked the empress's hand in marriage in a smart political move, but she had him killed. His son Rostam became furious, and decided to lead an army all the way from the north east to the capital, defeating every army Azarmidohkt threw at him along the way. He arrived and killed the empress, avenging his father. Borandokht was installed once again. After a short reign she died. The last Shahanshah of the Sassanid dynasty and last Zoroastrian emperor in Iranian history, Yazdgerd III, was crowned in 633 in a fire temple in the old capital of the Sassanid Empire, Estakhr. He inherited an almost disintegrated empire in complete chaos, one that was also incidentally being attacked by the Arab forces of Islam pouring out of the Arabian desert. This chaotic situation was an accumulation of centuries of Persian-Parthian antagonism, made worse by Khosrow I, Hormozd IV, and Khosrow II. The whole socio-political structure of the Sassanid Empire was teetering.
Now in order to fully understand the main subject of this thread, the fall of the Sassanid Empire at the hand of the Arab Muslims, we have to take the above into consideration which puts things in context and explains the main reason behind the ultimate Persian defeat in the Arab Conquests. When the Arabs first attacked the Sassanids Empire, it happened in the Battle of Ubullah in 628, shortly after the end of the Roman-Persian War and during the reign of the child king Ardashir III, when the Persian-Parthian confederacy was disintegrating and when the empire was consumed by chaos and divided into 3 different faction, a division that also extended to the military. This meant that the power of the central authority was non-existent. The military effort against the invading Arabs lacked determination, coherence, consistency, and strength. There was no concentrated and coordinated war effort against the Arabs, and therefore no serious attempt at putting an end to the threat. The "leadership" of the Sassanid Empire was fragmented, and decision-makers were busy vying for power and influence. To them, the invading Arabs were a secondary issue. Therefore, the task of defending the realm was mostly left to the forces stationed at the frontier and led by petty commanders.
4 battles were fought during the reign of Ardashir III (628-630): Ubullah, Dhat al-salasel, Madhar, and Ullays. All of them ended in Persian defeat. During the short reign of the usurper Shahrvaraz (630), 7 engagements took place between Persians and Arabs: Maqr, Veh Ardashir, Anbar, Ayn Tamr, Dumat al-Jandal, Husayd, and Firad. Again, Persian defeat was the outcome of them all. Then came the first reign of Borandokht (631) in which 3 battle occurred: Namariq, Kaskar, and Buwayb. As usual, Persian defeat in every engagement. The first and the last decisive Persian victory against the Arabs was achieved in the Battle of the Bridge during Borandokht's second reign (632).
All the previously-mentioned battles except the last one took place when there was no unified leadership in the Sassanid Empire, no decisive plan of military action. They occurred whilst the Sassanid factions were competing with each other for power, whilst Shahrvaraz had a vast army at his disposal doing nothing, whilst he was usurping the throne, and whilst Rostam was marching and fighting against the central armies of Azarmidokht. Considering the events that were transpiring in the Sassanid Empire at the same time when the Arabs were scoring one victory after another, the picture becomes clear. The victory in the Battle of the Bridge was only achieved because of unified Sassanid leadership. During the second reign of Borandokht, the leading figures of the Sassanid Empire decided to put aside their differences and decisively deal with the situation they found themselves in. This led to the first serious coordinated and concentrated Persian military effort against the Arabs, and the result was a victory on the battlefield in the aforementioned battle. But still, when the Arabs were retreating during the battle and the Persians were about to finish them off, according to Islamic sources news reached the Sassanid commanders (amongst them was Rostam) that there was a serious revolt back home that threatened to undermine the agreement between the leading factions. The Persians stopped their pursuit of the fleeing Arabs, preventing a massacre that could have had serious effects on the Arab war effort, though the defeat demoralised the Arabs and made them go quiet for nearly 3 years.
But that battle did not save the already doomed Sassanid Empire from ultimate destruction, because in 635 the Sassanid army was decimated in the Battle of Qadisiyyah, and the most powerful man in the empire at that time, Rostam, was killed in action. The path to Ctesiphon was opened for the Arabs as a result, the city fell in their hands. Since Rostam was now dead and there was no-one of similar status left in the Empire except his brother Farrokhzad, the Arab Conquest was greatly facilitated. After the massive defeat, the people were greatly demoralised, and it's not an exaggeration to say that they lost hope for ultimate victory.
The last serious stand of the Sassanid Empire was in Nehavand in 642. Because the Arabs had been victorious, the Sassanid military exhausted, and spirits were low, the Persians failed again, and with that failure the back of the Sassanid military was broken once and for all. Afterwards, the total Arab conquest of the Sassanid Empire was pretty much inevitable to the Iranian people. They began to either submit without a fight, fight without hope, or commit treason by defecting to the Arabs in order to save their skin (and to some defectors, their status). Even Farrokhzad, Rostam's brother whose task was escorting the fleeing Yazdgerd III, defected to the Arabs and left the unfortunate Shah alone without allies. Yazdgerd was killed in Marv in 652 by people who realised that the Sassanid dynasty was already a thing of the past.
Overall, the reasons behind the fall of the Sassanid Empire were varied. There was the disintegration of the socio-political regime mentioned above, the severe military, economic, and mental exhaustion from constant warfare (particularly caused by 28 years of armed conflict from 602 to 628), the plague that ravaged the lands, and the Arab religious zeal. The writing was on the wall, as they say.
Survivors of the Sassanid dynasty fled the realm and headed towards central Asia and China. We know that there was certain Piruz III who set up an exiled Persian court in China. His descendants assimilated and became pretty much Chinese.
People in mediaeval Iran mostly lost interest in Sassanid history, the Sassanids became irrelevant to them, though they did remain fresh in the memories of elites and learned men. It was the national poet Ferdowsi who reminded the people of the greatness of the Sassanid dynasty, since he immortalised them in Iranian national history by writing his epic poem, theShahnameh, or the letter of Kings. ....
