What central issue sparked the warfare eventually led to the decline of Rome?

Well, when Rome was at its peak in land, wealth, and power, a brief coup took place where a man named Diocletian took the throne by force and named himself emperor. His first act was to divide the empire in two, between himself and his colleague, supposedly because it had become to big to maintain successfully. He took the half east of the Italian Peninsula, and set up his capital in Constantinople (also known as Byzantium and Istanbul), while his friend took the western half, with Rome remaining the capital. Soon afterward, tribes from Germanic Europe and the new neighboring Hungarians decided to invade. While eastern Rome felt the success of Middle Eastern trade routes and thus had the money and stability to fend them off, weakened Western Rome was falling apart and was quickly divided among the barbaric tribes. After the death of Western Rome, Eastern Rome became the Byzantine Empire and thrived for another thousand years.