Reasons for its decline and fall

 After the death of Darius, his son Xerxes ruled until 465 B.C. Xerxes was a cruel but weak king who was also defeated by the Greeks in the Persian Wars. During Xerxes’ reign, the Persian Empire declined. Although the empire continued for more than a century, it grew weaker as it constantly faced conspiracies, assassinations, and revolts by the people who were burdened with heavy taxes. Alexander the Great defeated King Darius III and the Persian army in 330 B.C. Darius was subsequently assassinated by one of his own followers. Although Alexander retained the Persian system of government until his own death in 323 B.C. Darius’s defeat marked the end of the Achaemenid dynasty and the Persian Empire. 
Xerxes the Great 
The social structure started to break down once there was no longer a strong leader. The people below the upper class started organizing riots and other actions of revolt against the rulers. This caused the rulers to go against their religion by working for the evil spirits. Kings became greedy and began stealing from the supply if riches rather than giving it to the people.

 The lower class started riots against the rulers.
 Political- For one thing, weak rulers led to numerous provincial revolts, especially in Egypt, which always had detested Persian rule.  Secondly, the provincial satraps also became more independent, ruling their satrapies more as kings than as the king's loyal subjects.  They even carried on their own foreign policies and waged war on each other, which only added to Persia's problems. Revolts and unruly satraps caused serious economic problems for the empire. The government definitely grew weaker after the start of the fall of the Persian empire. The government struggled with thriving without having a strong ruler that could train armies to battle and gain wealth for Persia. The government started to shift to a very dysfunctional dictatorship. The Persian army was negatively affected because they no longer had a strong base or leader to keep them in line.  The emperors shrived to have total control.
 Intellectual- The Persian people no longer received the resources to build and sculpt beautiful artwork and buildings. This fall caused there to be much less production of pieces of artwork. We today have a gap in Persian art because of the fall. 
 Economic-Persian taxes became heavier and more oppressive, which led to economic depression and revolts, which in turn led to more repression, heavier taxes and so on.  The Persian kings also started hoarding gold and silver rather than re-circulating it.  This created economic turmoil without enough gold and silver for doing business.  As a result of this economic turmoil, the Persian kings got weaker still, which fed back into the problem of revolts and powerful satraps and so on. Crop production decreased as well which caused more commoners to leave and less trade to happen.   The taxes on the citizens grew.
 Scientific-  They could no longer make scientific advancements. Persia decreased in complexity towards the end of its era. The decrease in these advancements caused their weapon production and safety to diminish. This was detrimental to the empire and was a major reason why it fell.