The key difference between 146 B.C. and 400 A.D. vis-a-vis Roman power was that by the latter date Rome's system of taxation had all but collapsed. Even the most cultured and sophisticated nation on earth could not support an army without adequate funding.
Pas d'argent, pas des Suisses
The second big difference is that the cadres of the old Roman army were wiped out in the battle of Adrianople (378 A.D.) and never replaced. Instead of turning barbarian recruits into good Roman soldiers, Rome now had to resort to hiring barbarian tribes wholesale for defense. The results are plain to see.
Do you have any view about Mr. Hanson's claim that late Rome had a big problem with cultural relativism and cultural self-doubt?