Answer: They were successful because they took over the Italian Peninsula and they had a good military in place for some time, however their military did eventually not do them justice which did lead to the destruction of the Roman Empire. They were unsuccessful because of their split empire which caused the Roman Empire to crumble.
The most straightforward theory for Western Rome's collapse pins the fall on a string of military losses sustained against outside forces. Rome had tangled with Germanic tribes for centuries, but by the 300s “barbarian” groups like the Goths had encroached beyond the Empire's borders.
Rome became the most powerful state in the world by the first century BCE through a combination of military power, political flexibility, economic expansion, and more than a bit of good luck. This expansion changed the Mediterranean world and also changed Rome itself.
What were some weaknesses of Roman Empire? There was Political instability, which is when Rome never found a proper way to peacefully transfer polical power to a new leader. As well as Econmic and social problems such as financing the roman army. Trade suffered as well.
