The empire got divided. The East and West stopped viewing themselves so much as the same empire. Trade sometimes broke down. They didn't support each other militarily. The Eastern Empire was generally stronger, so it allowed invading tribes to focus on the weak point in the West. Some historians would say that the Roman Empire stopped expanding and it needed that expansion in order to keep getting more land and more slaves that really drove its economy. One major factor probably was the Huns that caused this great migration in the fourth and fifth centuries that caused the Germanic tribes to cross the Rhine and cross the Danube and become either, or either settle as part of the Roman Empire or threaten the Roman Empire.