When did the Roman Army decline?

[ValentinianVictrix]

My point is this...if the Romans were so capable of drawing those numbers why not at Adrianople? Obviously because border troops are incapble of mobilization on mass and often deserted and Valens' Comiatenses was in Persia/Armenian border. Valens had to run back and get as many troops as possible and not all them top tier for mobilization.

My main question is this....when did Border troops along with Field armies get into this bad of shape? Ammianus reports how most of the troops at Adrianople were super tired, unreliable, and charged without orders and the defeat was almost predictable. Compared that to the troops of Constantine who were super agressive and fanatical at the Battle of Chrysopoplis.

Speaking about army sizes...what was the total numbers of Trajan's invasion of Dacia and Persia? And Severus' in Persia?

About Roman cavalry, their role seems to be almost the same, for light attacks and skirmishing as proven in Aurlian's campaigns. Their main difference was strategic as 1/3 of regiments were now cavalry. Cataphracts don't seem to have a huge preformance difference, as they are often slow and prone to get attacked from sides from infanrtry. How does Roman cavalry compare to the Germanic and Sarmatian Foderetii? Sarmatian soldiers seem to be already in the army since Hadrian. Did they loose their horseriding ablity when they gained citizenship?

I am not sure why you believe the Roman army at Adrianople were in your words ' super tired, unreliable, and charged without orders and the defeat was almost predictable' Because that is not what Ammianus actually wrote. This is what Ammianus said about the army at Adrianople-

'He (Valens) had under his command a force made up of varying elements, but one neither contemptible, nor unwarlike; for he had joined with them also a large number of veterans, among whom were other officers of high rank and Trajanus, shortly before a commander-in‑chief, whom he had recalled to active service.' Amm XXXI, 12, 1.

The army at Adrianople contained a number of crack legions, including the Lanciarii and the Matiarii, plus a number of crack Auxila units including the Batavi and probably the Heruli, who were normally brigaded with the Batavi, and the Regii and Victores. He also had some of the Late Roman Empires most experienced military officers in his army, officers such as Richomeres, Victor, Sebastianus, Trajanus, Aequitius etc.

All the evidence points to Valens actually not planning on fighting a battle that day, it really does appear that Valens went to the Gothic camp to conclude treaty negotiations with the Goths before returning to Adrianople before nightfall (hence the reason why he did not take his baggage train with the army and leaving 'a suitable guard of legions' behind in the camp outside the walls of Adrianople instead of just moving the officials and treasury inside the city and taking the legions defending the camp along with him, and he did not stop a short distance from the Gothic encampment to build a field camp as he should have done had he actually been planning on fighting a battle because that is where he would have retreated to had he suffered a reverse).

If you have a situation where your army standing in the sun and suffering from thirst suddenly being attacked by previously unseen Gothic cavalry who proceed to drive away both your cavalry wings, then I suspect then defeat is almost inevitable!

Whilst indeed Roman cavalry still were tasked with defending passes, setting ambushes on roads going through concealing terrain etc, they also fought hand to hand against the opposing cavalry as the situation dictated. The Catafractarii were instrumental in helping the Roman infantry who Valentinian had tasked to ambush a band of Saxon raiders but which infantry then revealed their location too quickly and were almost overun by the Saxons until a troop of Catafractarii who were nearby, on hearing the commotion took the Saxons by surprise, allowing the Roman infantry to rally and the combined force wiped out the Saxons to a man.