Similar Questions for How did Justinian Gain The Throne from Yahoo AnswersQuestionAnswer
By the time of the First Crusades, the Byzantine Empire had reached its peak.
The Empire was nothing more than just one general using his army to overthrow the emperor and then proclaim himself as emperor.
The emperors were no longer the great generals of Justinian, who had vision and drive. The emperors of Byzantium had grown fat and lazy on power.
The emperors spent more time using the military to defend the empire against other Byzantine generals and threats from the Christian crusaders as well as the muslim invaders.
To gain suport for their bid for the throne, Byzantine generals would make promises and give bribes for support to other byzantine generals as well as muslim tribes and then repay the bribes by giving away parts of the Byzantine empire.
The corrupption was so rife that the emperors could no longer afford the up-keep of the city's defences. The main wall of the city desperately needed maintenance.
Centuries of in-fighting, civil wars, corruption and bribery eventually made the empire so weak that all that was left was just the city of Constantinople.
There are many reasons why the Western Roman Empire fell.
The most important of these was the fact that the Western Empire had not had a functional and intelligent ruler since Constantine the Great, and he divided the Empire and moved east during his reign. This left Rome governed by a series of Generals who were more concerned with fighting each other for the throne then with actually steeming the tide of the problems assualting Rome. Had Rome had an effective Roman ruler, the Western Empire might have survived politically, although it probably still couldn't avoid many of the cultural changes that occured in the fifth and sixth centuries. But, Rome had no effective rulers, and this weakness made every other problem worse.
The next major problem was that all the German tribes migrated into Rome at once. The army of the fourth and fifth century could have managed to defeat each of the invading tribes, but that would only be if they came one at a time. The fact that they all came at the same time, thinned an army that was already dangerously over extended. Even if Rome had no other problems, Rome would have been reduced to at least the Italian peninsula because of the numbers of barbarian invaders assualting the Western Empire's borders.
The next major problem was the decline of the Western economy and its connection to the Roman military. Roman economy was based on trade, particularly with the east, including far off China. While trade functioned well and wealth came into Rome, Rome had the funds to raise a large professional army with maybe a few mercenaries as auxilaries. By the fourth and fifth centuries, outside of Rome, there were few major cities and trading centers in the Western part of the Empire, which made the West dependent on the East to keep its head aflout. When Constantine divided the Empire on his death, the West lost the funds from the East and its economy quickly tanked as it didn't have the trading centers to maintain any sort of wealth needed to raise a trained army. As a result, the West heavily relied on mercenary forces which continuously Germanized the Roman Army, which leads into Rome's next major problem...
The Germanization of the army actually weakened Rome militarily. While they were brave fighters, and by the fourth century had begun to adopt Roman tactics, they were more loyal to their tribal kings then they were to the Roman Emperor. As a result, German Generals, like Alaric, the Visigoth, Clovis the Frank, and Odoacer, the Heruli would gain power over the weak Western Emperors and by 476, Odoacer had replaced the last Western Emperor, Romulus Augustulus and by 500 Clovis united the Franks and conquered the last piece of territory ruled by a Roman in Gaul.
The last major problem was that the Eastern Roman Empire refused to spend its money to help the Western Empire. Many Eastern Emperors could have sent troops to bolster the Western Empire until the crisis in the west could have stabilized, but they didn't. The only Emperor to do so was Justinian in 517, by which time the Western Empire had fallen.
If I'm correct, his army took control of Constantinople.