When did the Roman Army decline?

[ValentinianVictrix]

I fully admit that unlike the Crisis of the Third Century, where combined armies of Goths and other Germanic's plundered and ravaged throughout Greece, the southern coastline Black Sea provinces, the provinces bordering the Rhine and Danube, even down as far as Rome itself in Italy, and the Parthians followed by the Sasanids were overrunning large areas of the Eastern provinces, the situation after Adrianople appeared less serious. Why the Romans were unable to eject the Goths from within their borders during the 4th & 5th Century AD, like they had during the previous century, has not been fully explained to my satisfaction. One plausible reason was that the Goths were still needed for future campaigns against the Sasanids and then possibly to aid against the Huns. Of course the Goths could not return to their former homelands as the Huns had now taken possession of them, which was a far different sitution than the previous century.