When did the Roman Army decline?

[Robert Vermaat]

I never said barbarianization was a reason for decline, the fact that the Romans can ONLY draw people across their borders proves their decline. The Auxilia were proffesional troops, Foederetii are not.

Who ever said that the Romans could only recruit across the border? That’s nonsense.
You mean Foederati, not foederetii.

Yes, Valens did have his field army in Persia, but you have to realize the Roman army was 500,000 strong at this time well at least on paper. The fact it was only on paper is a clear indication of corruption, militia army, and an inablity to mobilize to defeat the Goths.

Every army is smaller than it’s paper strength. Every army ever. It’s a load of nonsense to call that a sign of corruption. Nor does it has anything to do with militias or mobilization ability.

Field armies were divided up so I do not suspect Valens' Persian field army to be more than 100,000 in the East. The fact that he could only draw 20,000 in Thrace just show the manpower shortages.

You are constantly confusing the field army with armies on campaign. Constantine had a force of (reputedly) 100.000, but everyone assumes his actual field army no stronger than 30.000.

Again right after Adrianople, the Romans never bothered to muster an army to defeat the Goths in a decisive battle even when losing only 15,000 men. At the Battle of Constantinople the Goths were reported to be "outnumbered" yet the Romans could not finish off the Goths.

A matter of strategy. You seem certain they would have won, well, you were not there at the time. The loss of Adrianople must have influenced the general accordingly, and they did not dare risk another battle. It’s easy for you to judge that. But it does not say anything about corruption or not.

Roman armies in the Principate from Aurelius and the 3rd century Crisis was capable of having the quality Legions to deploy in Africa, Germania, Thrace and Persia, the Late army after Julian was totally incapable. The days of the Legions of Marius were the Romans could draw mass ammounts of reserves from its civilians was over and suffer 80,000 complete annihalations and still recover....was over.

The Romans could still draft many soldiers, but the landscape was indeed different. Although far more Romans were citizens, the frontiers that had to be guarded were far longer. The Roman Republic, given the amount of troops and the length of borders in the 4th century, would also have had great difficulties to raise an army after the defeats against Hannibal.
If you make comparisons, please DO make them on good grounds.