What was the reason behind the fall of "Roman Empire"?

In the 18th century Gibbon wrote a history of the Roman empire in two massive volumes, which account is still adhered to today because of its rigorous research and scholarship. Gibbon believed the empire eventually fell due to moral decadence, perhaps having Gomorrah in mind.

He thought that the ideals of political and intellectual freedom that had brought such cultural richness to the western world had dissipated into a mere gluttony of worldly delights. I suppose you could interpret this as a taking of good without any giving of good. Gibbon's account is of the ethics, discourse and attention to human development being frittered away by dilettantism.

The Roman Empire was an area of Europe and Asia and it took place from 27 BC until 395 AD. So, it took place for a large period of time. Constantinople was the largest city later in the era of the Roman Empire with Rome being the largest during the earlier times. This empire was ruled at times by Julius Caesar and Caligula. There were some major events that took place during the Roman Empire.

This includes Hannibal invading Italy and the Laws of the Twelve Tables. Even though the Roman Empire lasted for many years and was once the most powerful empires in history, it came to a crashing end when people stopped following the orders of their leader. Due to its vast space, it was very difficult to rule all of that land.