The downfall of Rome came from the invasion of several tribes that were taking advantage of the overextension of the Empire or were escaping the Huns.
These invaders were usually referred to as Barbarians which the very name implies lack of civilization or integrity.
But how uncivilized were really the Visigoths, Ostrogoths and Vandals (among others) when they attacked Spain, Rome, and Africa? Were they really as depicted in the media like bearded, shirtless men living in wooden huts and having no organized warfare?
I would accept this description for barbarians in the Roman Republic, since tribes were still widespread and technologically underdeveloped. But 400 years later, and with larger, civilized empires having risen and fallen, couldn't these tribes have actually organized better and lived in better conditions?
After all these tribes ended up converting to christianity and became civilized enough to found the kingdoms that became Spain, Italy, France and the HRE, among others.
Sure enough they raided and sacked, but the Romans and Greeks also did that, and I don't think they were as murderous and aggresive as the Huns were at that time.
It's actually, I believe, a common misconception that "barbarians" raided Rome from the outside of the empire. Certainly, the Goths came from outside the Empire (I don't believe there's a consensus on exactly where, perhaps an expert can clarify/correct me). However, they were incorporated into the empire as foederati, soldiers who fought as auxiliaries in the Roman armies. In fact, the Huns were foederati for a time, as well.
Anyway, these were people being incorporated into the empire. When Alaric was named "Rex gothorum," it was a title that had not existed before. It was invented by Goth foederati in the Roman Empire. Alaric proceeded to sack Rome, but his men were not shirtless barbarian hordes, they were trained soldiers who had lived within the Empire for generations. In fact, most had converted to Christianity, albeit Arianism, which was the wrong "brand" of Christianity at the time.
The Ostrogoths, too, became Roman foederati. Theodoric, a leader of the Ostrogoths, was sent by the Eastern Roman Emperor to depose Odoacer in Italy. The Ostrogoths were not a barbarian horde (nor was Odoacer), they were sent on a campaign to Italy on the orders of the Emperor.
So, these people were not outside barbarians. They lived in the empire for centuries and became incorporated into it over time. Those invasion maps you see in high school with arrows coming from outside the empire are completely false