The invasion by Alexander king of Macedon, and their inability to match his strategy, tactics and armoured soldiers.
Some interesting history to study. The world moved on after the fall of the empire.
darius used some of persian's great wealth to build roads across the empire
From carpenters to the king, the Persian Empire had very similar jobs to that of any other empire. (i.e. farmers, artists, servants, etc)
he had a giant zit
Lack of communication throughout the empire/society/etc. would definetely be one of the major difficulties to taking control of a large empire/society/etc. Another would be travel. "How would they get around?" would be a good question for that. Simple difficulties such as these can lead to the fall or decline of an empire,nation,etc.
At its peak it stretched from Egypt to central Asia
Some of the Greek city-states joined with Persia, the southern cities opposed the Persian attempt to absorb them into the Persian empire.
The decline of Rome dovetailed with spread of Christianity, and some argue that the rise of the new faith helped contribute to the empire's fall.
The Middle Ages (the term Dark Ages has been out of favour for quite some time) was the period which followed the fall of the Roman Empire. Therefore, it no longer existed.
Kings Cyrus the Great, Cambyses II and Darius the Great formed and consolidated the Empire.
Government problems
What best describes the Persian Empire is an empire that we're very strong and fought many battles. Also they we're an empire that had an enormous empire! The biggest of the time, that stretched over the vast lands of Turkey, Israel, Jordan, Iraq, Iran, Afghanistan, Pakistan and a little tiny bit of India. They also had some magnificent leaders, that rulers for many years. So obviously the Persian Empire was a great period of history.