Why did the Western Empire Struggle for Survival?

The most obvious and perhaps most prominent of the reasons behind the Western Empire's struggle for survival is perhaps the German invasions. United under Alaric the Gothic war leader, and tired of living under "the pressure of Roman disdain"  the Visigoths seek to take back "what they thought they deserved" (Wise Bauer pg 74) of the Roman Empire. The Visigoths spend considerable time reaking havoc in Eastern Rome. However their most lasting impact is the harm they contributed to the Western Empire specifically Rome, which weakened Roman military, resources, and morale (Wise Bauer Chapter 11). However Rome had stood undefeated for nearly 8 centuries (Wise-Bauer pg 83) surely no united band of barbarian tribes should have so easily destroyed the walls of this 'eternal city'. I believe the problems of Rome go much deeper than the raids of the Germanic tribes. For raids and battles are outwardly tangible, I believe Rome's wounds are inward and go so deep to the bone. For even the historian Wise-Bauer agrees that in 330AD Rome was a "splintering empire" in need of unification (Wise-Bauer pg. 11). I would have to agree with Augustine that once the threat of enemies was destroyed (around 150BC) "the extinction of that threat (Rome's enemies) [was] immediately followed by disasters arsing from prosperity" Augustine goes on to say: "so that the Romans who in a period of high moral standards stood in fear of enemies suffered a harsher fate from their fellow citizens when those standards collapsed . And the lust for power, which all of human vices was found in its most concentrated form in the Roman people as a whole," (Augustine Book I, Chapter 32) For Augustine then claims that after the sack of Rome "nations in the east were bewailing your catastrophe [... but] you [the Romans] were asking your way to the theatres [...] behaving in a much more crazy fasion than before." For even great Scipio knew that "it was this coruption, this moral disease, this overthrow of all integretiy and decency that" he, Scipio, dreaded, for  "[Scipio] saw how easily you [the Romans] could be corrupted and perverted by prosperity [... if you, the Romans were] relieved from the enemy's threats. He did not think that a city is fortunate when its walls are standing, while its morals are in ruins" (Augustine Book I Chapter 33)I would have to stand with both Augustine and Scipio that Rome fell not in 410, but rather it fell during the Punic wars when its enemies were defeated and when Rome then turned away from 'high moral standards' and toward 'the lust for power' and immoral living. Rome's loss of morals is equivalent to Rome's loss of power.

City of God, Book 1, 10

Chapter 10 of City of God  Book 1 deals with the distinction between 'possessions of the outer man' vs. 'possessions of the inner man'. In this chapter Augustine makes the claim the Christians "who loose their earthly riches in disaster" really  loose nothing because they had in fact already "possessed them [riches] in the spirit" . However if people did love "goods from this world" more than 'spiritual goods'  then, states Augustine, God uses trials or 'the barbarian invasions' to "convey the lesson that what is to be loved is the incorruptible good".
The larger scheme of City of God is to take up "the task of defending the glorious City of God". It is a polemic whose purpose is to show "those who prefer their own [Roman] gods"  that Christ is not at fault for the sacking of Rome. Chapter 10 fits into this 'larger scheme' because it proves how the sacking of Rome or the barbaric invasions really did not rob any people of true and 'incorruptible' possessions, but rather the invasions taught people a valuable lesson.