When did the Roman Army decline?

[ValentinianVictrix]

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.

In December 1944 the population of the UK was just over 40 Million, the population of the USA was approximately 135 million. Yet, despite this both nations field armies were almost at breaking point when the Battle of the Bulge broke out on 16th December of that year. Its been estimated that the UK only had two regiments worth of reserves, the USA about 12, if the Germans had reached their objective of Antwerp then the Western Allies would have had to negotiate a peace.

So, its not your population levels that count, but the numbers you can train in time to be combat effective. And recruits literally do not grow on tree's, you have to replace men through recruiting the able, then the old and young and then finally you have to rely on increased birth rates to provide the next generation of soldiers. It can take decades to build a nations army up to full strength.