The fall of the roman military power

[Robert Vermaat]

Hi, theres a question thats been buggin me about the late army
By 180AD by the time Marcus Aurelius had the war in Germania with the different barbarian tribes the roman army was a killing machine but slowly that machine became weak. At what point did this happen were the reforms made by Constantine what killed the efficiency of the army or what was it , was it the influence of the barbarians in society , what happened ? What made the once invincible roman army weak and beatable

two things.

One, civil war. that blead the Empire, killed off too many soldiers, making it impossible to kep up standards.

Two, changing tactics. Being nearly unbeatable in the field, the enemy changed tactics both of the European as well as the Asian fronts. We see the Germans shifting to small raids, ambushes, surprise tactics. The Persians use scorched earth tactics, because even when it came to pitched battles the Romans could beat them.
Another changed tactis was the deep raid, also difficult for the old style army.

Reacting to that was not possible for a legion, nor an army organised the way the 'classic' army was. Therefore you get the 'defense in depth' and the development of the Mobile reserve (already under Gallienus, probably, not Constantine) and the gradual re-development of the army into Limitanei and Comitatenses.

Oh and, where did you get that 'Hollywood'-notion that the Legions were unstoppable and the Late Roman army wasn't? The latter was probably far more versatile, but facing stronger opposition and a worse economic situation, that was nonetheless the army that went under.

Sources:
Luttwak, E.N., The grand strategy of the Roman Empire (Baltimore 1976) 255p.
Nicasie, M.J., The twilight of empire. The Roman army from the reign of Diocletian until the battle of Adrianople (Amsterdam 1998)321p.