Changes in the Roman Army

[Flavivs Aetivs]

I must ask, from your original point, what you believe (in that particular microcosm) changed so much that it would influence a set of changes for the Late Roman period (4th-5thC)?

Cavalry.

Rome necessitated its changes because it fought the Sassanids, who unlike the disorganized Parthians could coordinate large armies and effective defenses, as well as invade (And at one point almost conquer) the Roman Empire.

And then came the Huns, with a completely new style of warfare and the 7-lathe bow, revolutionizing Roman cavalry by the mid-6th century.