When did the Roman Army decline?

[Mrbsct]

I just don't get how the Late army of post-Julian is somehow the "same quality" when you say "lack of manpower" or "recorvering". 30 million people and they can barely even recover the manpower, relying on Foederetii. That is nothing compared to even the 3rd century Crisis where entire armies were lost. The decline is due to politics, economics, military defeats, and civil wars. My question is what event(like what Civil War?) that caused it?

As far as Adrianople, the Elite Schoe Cavarly literaly broke ranks without any orders and encircled the Gothic encampment only to get driven off easily by the Gothic calvalry.