What factors led to the transformation, decline, and fall of the Roman Republic according to Livy and modern historians?

Invasions by Barbarian tribes were a major cause of the decline of the Roman Republic. Although these tribes had been a constant source of conflict, things changed, as they were able to encroach and breach the Roman boundaries. Other causes include the division on the Empire into two halves (Eastern and Western), economic problems, religion, and the ineffectiveness of the army.