Why did the Achaemenid Empire Fall?

Throughout time there have been a number of Persian Empires, but none of them can compare to the great Achaemenid Empire, which ruled between 550 to 330BC. The Achaemenid Empire is known as the largest empire in Ancient history which stretched out approximately 8 million km² at the height of its power. So how does an Empire so large and with such great power collapse? Was it struggle for power, which every new king had to suffer after the death of Darius the Great? Or was it because of corruption of the ministers and Satraps that made the empire decline. Maybe it could have been the invading Greek forces lead by Phillip the II of Macedon and his son Alexander the Great or are all these to blame?
Xerxes the I had to fight for his rule his elder half brother Artobazan, who was considered illegitimate to the throne because of his commoner mother. Xerxes was murdered in 465 BC many theories speculate who murdered him but it is certain that has is certain that he was killed intentionally. He was succeeded by Artaxerxes the I who killed 2 of the 3 of his brothers to gain the throne. Artaxerxes’ son Xerxes the II was assassinated a few weeks of coming into power by his half brother. Another of his half brothers, Darius the II took power and had the assassin executed. Darius the II was succeeded by his son Artaxerxes the II. Cyrus the younger, Artaxerxes’ brother, was plotting to assassinate him so he was arrested but was not executed and was sent to Lydia as Satrap. Cyrus prepared an army to rebel against his brother and they fought at Cunaxa where Cyrus was killed. Artaxerxes the III is next to the throne, he murders 8 of his brothers to ensure his place on the throne. He is poisoned by a minister named Bagoas and his successor Artaxerxes the IV is also poisoned by the same minister. Bagoas places Darius the III on the throne. These assassinations and treachery affected the leadership of Empire. With many of these Kings paranoid with the possibility of being assassinated, they neglected the running of the Empire. Also the time between the deaths of the kings was full of tension, these two factors caused