This is the subject of a great deal of argument. Some theories:
- Gibbon says it was internal disagreement caused by the rise of Christianity.
- Some argue that they deforested and otherwise ruined their environment until it couldn't support so many people.
- Class conflict. Some argue that the people in the Army were responsible for keeping Rome great. But because they were poor they had no political power. So they got angry and kept rebelling.
- Some say that Rome just got too big and expensive to run and defend effectively.
This is an important and great question. Perhaps, it is best to start this post by stating that it might be helpful to ask a different question. Why did the Republic last for so long? Even during the turbulent times of the Republic, the elite class tried to follow Republican forms. This is Erich Gruen’s basic point in his magisterial book, The Last Generation of the Roman Republic. With that said, one has to come to terms with that fact that things did change. At one point the Republic was a distant memory and the Empire and emperor were realities. What happened? With such a huge question, there were many variables. This can be likened to a twenty-inning baseball game. A hit there, a double play here, or timely slide could have changed the outcome. However, if I were pressed to give an answer, I would say that the Roman magistrates got more and more powerful. Also they developed special relationships with the army. This inevitably led to civil conflict. This changed everything, in my opinion. Civil war, especially multiple civil wars, have the ability to change society, even in radical ways. The outcome was that people were shattered by these wars, land dispossessions, and other evils. What they wanted more than a Republic was peace. When Octavian came into the picture with incredible political skill and a promise of peace, things changed forever.