The Western Roman Empire fell when the western romans were not any more nationalistic. They would no longer serve in the roman army. They were not willing to die for their patria. When the romans would stand as one man and fight for their city, putting their lives on the line, they could and did win battles against enemies outnumbering them by factors of 10. They built up an empire that until modern times had not been equaled in the west. But then they got faggy and wouldn't fight anymore. They were content with their free food rations and entertainments at the coliseum. The got the foreigners to do their fighting for them. And the foreigners eventually took over.