Late Roman Army during the 5th century

[starman2012]

The difference was that the Empire had the massive infrastructure of Gaul and Italy to maintain, along with a professional army rather than a Militia Army. In the republic you supplied your own equipment while in the Late Empire massive fabricae supplied rather standardized equipment for the whole army.

I think the main difference was a reluctance to serve, for whatever reason(s). Massive infrastructure and fabricae had been in existence for some time, and professional armies go back to Augustus. The Empire seemed resilient enough with all these down to 300 CE at least.

Not only that, but in the Republican Era most of the Farming was in italy - in the 3rd century agriculture began moving to Syria and Proconsularis/Numidia/Tripolitania.

From what I've read, Egypt had already become the granary of Rome in/by the first century. Not only was Vespasian said to have gained an edge by "capturing Egypt and controlling its grain supply" the original deployment of legions in Syria was said to reflect concern for the safety of Egypt, given its importance. I don't perceive a correlation between reliance on nonItalian agriculture and military impotence. The Empire was pretty strong most of that time.