Answer:
Hired mercenaries: Emperors got lazy and decided to start hiring mercenaries (paid soldiers) to fight in the legion. However, mercenaries lacked the loyalty and discipline that a Roman citizen had, and were often paid even more by enemies of Rome to fight on their side instead. Also, once they were done fighting for the Romans, they might go to another empire's army and teach them all of the tricks that they learned from fighting with the Romans.
Step-by-step explanation: