Why was it that the west had fallen into political fragmentation while the east remained rich, powerful, and united? There are a few major reasons:
First, Constantinople itself played a major role in the power and wealth of the east. Whereas Rome had shrunk steadily over the years, especially after its sacking in 410 and the move of the western imperial government to the Italian city of Ravenna (which was more easily defensible), Constantinople had somewhere around 500,000 residents. That can be compared to the capital of the Gothic kingdom of Gaul, Toulouse, which had 15,000 (which was a large city by the standards of the time for western Europe!) Not only was Constantinople impregnable to invaders, but its population of proud Romans repeatedly massacred barbarians who tried to seize power, and they deposed unpopular emperors who tried to rule as military tyrants rather than true emperors possessing sufficient Roman "virtue."
The East had long been the richest part of the empire, and because of its efficient bureaucracy and tax-collecting systems, much more wealth flowed into the imperial coffers in the east than it did in the west. Each year, the imperial government in Constantinople brought in roughly 270,000 pounds of gold in tax revenue, as compared to about 20,000 in the west. This made vastly better-equipped, trained, and provisioned armies possible in the east. Furthermore, the west was still dominated by various families of unbelievably rich Roman elites who undermined the power, authority, and financial solvency of the western imperial government by refusing to sacrifice their own prerogatives in the name of a stronger united empire; in the east, while nobles were certainly rich and powerful, they were nowhere near as powerful as their western counterparts.
There is another factor to consider, one that is more difficult to pin down than the amount of tax revenue or the existence of Constantinople’s walls. Simply put, Roman identity - the degree to which social elites, soldiers, and possibly regular citizens considered themselves “Roman” and remained loyal to the Empire - seems to have been stronger in the east than the west. This might be explained by the reverse of the “vicious cycle” of defeat and vulnerability described in the last chapter regarding the west. In the east, the strength of the capital, the success of the armies, and the allegiance of elites to Rome as an idea encouraged the continued strength of Roman identity. Even if poor farmers still had little to thank the Roman state for in their daily lives, their farms were intact and local leaders were still Roman, not Gothic or Frankish or Vandal.
Lastly, the east enjoyed a simple stroke of good luck in the threats it faced from outside of the borders: the barbarians went west and Persia did not launch major invasions. The initial Gothic uprising that sparked the beginning of the end for the west was in the Balkans, but the Goths were then convinced to go west. Subsequent invasions from Central Europe were directed at the west. Even though the Huns were from the steppes of Central Asia, they established their (short-lived) empire in the west. Eastern Roman armies had to repulse threats and maintain the borders, but they did not face the overwhelming odds of their western Roman counterparts. Finally, despite Persia’s overall strength and coherence, there was a lull in Persian militarism that lasted through the entire fifth century.