The collapse of Western Roman Empire is a very classic topic among scholars of history, politics, economy, etc. Joseph Tainter had a theory, based on complexity and energy management.
If you have time, you can watch this lecture by Joseph, it's a fun and somber talk about the collapse of Western Roman Empire from his perspective. I also read the book The Collapse of Complex Societies, which elaborates his theory of social collapse with more examples than just the Western Roman Empire, including the Mayan Empire and Chacoan civilization.
The Collapse of Western Roman Empire
In the early days of Western Roman Empire/Roman Republic, it conquered a lot of new lands. Each conquest was easy, and brought great loots. The Romans used the loot to increase their wealth and power and social complexity.
Some complexities were voluntary: they wanted to enjoy life. Some complexities were forced on them: they needed it to keep their growing republic/empire in order. These complexities were mostly in the forms of bureaucracies and law codes.
Eventually, the conquests ground to a halt: its borders reached the Germanic, Persian, Briton, and so on, countries that were either too powerful to conquer or too poor and distant to worth conquering. Rome went from conquest mode to farming mode. This placed a great burden on the empire: the economy and culture used to run on conquests, but now it has to run on farming.
