So, interestingly, it actually was able to defend itself right up until the end. The Roman army of the fifth century retained its ability to find success in large battles. Some examples are Stilicho's defeat of the massive Goth army, led by Radagaisus, in 405. Constantius defeated Gerontius and the usurper Constantine III, in around 410. As late as 451, a mere 25 years before the final Roman emperor in Italy would be deposed, the Roman general Aetius won a massive victory over Attila the Hun. Majorian defeated the Visigoths in 458. So the western Roman armies were still having major successes against their enemies in large and significant battles.

Also, many of Rome's losses and defeats in this period were from deceit and trickery, or largely uncontested invasions. For example, Alaric's sieges and eventual sacking of Rome which occurred from 408 to 410, went largely unopposed militarily, due to the recent execution of the preeminent western general, Stilicho. The 406 invasions across the Rhine, which would carry barbarian groups all the way through Gaul, Hispania, and across North Africa, went mostly uncontested by major armies. The invaders would have encountered the limitanei, the border forces, but those did not have the numbers to stop major invasions. When these invasions were contested by the larger field armies, they were slowed and halted, such as when forces reinforced Carthage and forced the Vandals to agree to remain in Numidia - but that didn't last long! The major defeat at Cap Bon in 468, was a naval defeat brought about through deceit, not an actual engagement between armies.

So, overall, the late Roman army was still very effective. But how could an Empire have an effective army and still collapse? If you're asking that question, have a look at the faq and the profiles of some of the flairs who have answered questions there, there are some very good write ups about how the Empire didn't really collapse. It was more of a slow dissolution and change, where the provinces slowly but steadily grew more distant from the imperial center, eventually to a point where we consider them separate. There was no major and decisive battle that defeated the Empire. In fact, it was an element of the Roman army that deposed the last Roman emperor in Italy in 476, and appointed one of their own as king instead. The other elements of the army were absorbed into the successor kingdoms or created their own successor state, such as the Kingdom of Soissons.