The division between the Western and Eastern Roman Empires began in the 1st Century. During the Pax Romana the Eastern half of the Empire became more Hellenized. The Western half was Latinized. This trend continued through the reign of Constantine I (Lensing History 2). Constantine moved his capital to Byzantium, renamed Constantinople, signalling the shift toward Orthodoxy, Hellenization and the Eastern Empire.

The fall of the Western Roman Empire is attributed to multiple causes…

Attacks by the Sassanid (last Persian) Empire and Constantine’s mismanagement of funds created an potent economic crisis. The crisis continued for centuries resulting in degradation of local, or municipal, government agencies and centralized power. The development of a ruthless kleptocracy resulted in the loss of public support.

Official cruelty, supporting extortion and corruption, may also have become more commonplace; one example being Constantine’s law that slaves who betrayed their mistress’s confidential remarks should have molten lead poured down their throats. While the scale, complexity, and violence of government were unmatched, the emperors lost control over their whole realm insofar as that control came increasingly to be wielded by anyone who paid for it. Meanwhile, the richest senatorial families, immune from most taxation, engrossed more and more of the available wealth and income while also becoming divorced from any tradition of military excellence – Wikipedia

The 4th Century was characterized by a series of civil wars and invasions by Goths, Visigoths, Franks and Huns. Pirate attacks on Roman cities and bands of thieves were insufficiently handled by Roman armies. The Goths actually integrated into Eastern Roman society but were often a source of civil strife in the West.

The 5th Century the end of the Western Roman Empire. Alaric‘s sack of Rome in 410 was a humiliation to the remnants of Roman armies. The Western Empire temporarily ceased to have an effective military force after this invasion. In 457 Marjorian attempted to revive the Western Empire but by 476 AD the region had devolved into smaller states and fiefdoms.

…but there is really only one.

The Eastern Empire survived this period and is currently referred to as the Byzantine Empire. The Byzantine Empire’s survival while the Western Empire failed is worth considering. The various leaders trying to wrest power and leading invading armies were seemingly unrelated. Were they really? The macro view might lend the notion of a symphony of efforts to destabilize the Empire that Constantine I abandoned. A death from outside and a death from within, coordinated on another level. Had the people of the Western Empire perceived that the giants weren’t really gods? They selected a new empire and destroyed the old one behind them.