The fall of the Roman Empire generally leads to Constantine. Not only did he move Rome's capital to Constaninople but he allowed Christianity to become the dominant religion.
After Christianity became the dominant religion, Romans were more concerned with religion and the afterlife than governing their empire. They started hiring more and more barbarian mercenaries to fight for them, who often ended up turning on the Romans creating more problems. As their borders were weakening and their economy started to collapse, the Germans started moving in, and a mere 200 years after Constatine the (Western) Roman Empire had fallen.







