From my understanding of the imperial period of Rome and the time after the reforms of Marius, the organization of the army changed. It essentially became a professional fighting force. The generals of the army awarded soldiers with booty and plots of conquered lands. This shifted the loyalty of the soldiers from the state of Rome to whoever their commanding general would be as all rewards came from the general and not from the state. This explains Rome's many civil wars in the later periods. Again, this is just my understanding and someone with more knowledge may have a much better answer.