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I could be wrong but I am pretty sure I read that in Eastern Roman Empire they would use mercenaries to make up most of their army. When the Huns attacked they just gave the mercenaries money so they would either not participate in the battle or slow down so the don't make it to the battle.
Wasn’t one of the downfalls for the Roman Empire the creation of private armies that only answered to the paymaster and not the emperor? >

The Republic, specifically? Sure. The Marian reforms basically ensured that the legions were loyal to their commander, as he was responsible for their pay. The Senate was notorious for not paying the soldiers on time (if they bothered to pay them at all). That system pretty much guaranteed that the legions didn't give a shit about the Senate (and thus the republic) because their financial hopes rested solely upon the skill of the man commanding them.
The empire? More specifically the Western empire in the 5th century CE? A lot more complex. In that case the foederati ("allies", basically barbarians who were allowed to settle within the borders of the empire in exchange for military service and a degree of autonomy) had... questionable loyalty to the empire and the emperor, either due to issues with pay or simply due to Roman/Imperial prejudice that often lead to cold or hot hostility between the emperor and his "subjects".
It kind of became a moot point as the power of the Western emperors waned, however. The Western empire doesn't so much collapse in one catastrophic event as it does slowly fade out of relevance leading to its existence being essentially only nominal in large swathes of the provinces. (For example, the first titular king of the Franks, who were foederati themselves, was Clovis I, and he had, early on, conflicts with the nominal Roman "governor" of Gaul at the time. It's apparent by Clovis' time that Imperial power had waned so significantly in Gaul that the concept of the empire was really more an idea than a reality.) Foederati did absolutely revolt and run rampant through both the East and West at times, but they were only capable of doing so due to the overall degradation of Imperial authority, and even then they weren't always hostile. The Gothic foederati were crucial in the Roman victory over Atilla, for example.
So, really, I'd say the appropriate anology here would be the fall of the republic, not the empire. Hopefully the Russians are going to speedrun that shit and get the fuck out of Ukraine ASAP.
(Sorry that this post is so long. I'm a history nerd.)