Explanation: During the Late Roman Empire, when everything was falling apart and the Empire no longer really functioned in a coherent way, the massive, infamously militarily powerful Roman state increasingly opted to pay barbarian tribes to fuck off instead of fighting off their invasions.
... this rarely worked out.
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The barbarian tribes largely were not invading in search of money, so any such agreement didn't solve their fundamental problem of "We have no land and the other nomads behind us are going to enslave us." Which meant that sooner or later, and likely sooner rather than later, the crisis would repeat itself, requiring a new bribe or a defense against the tribes.
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Money is power. Giving someone who has expressed interest in invading you more power is not a fantastic idea.
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The Empire itself was increasingly broke at this point due to the ultra-wealthy evading taxes and the poor becoming poorer, and more taxed, in contrast to the rich. The poor also became less interested in the defense of a polity that increasingly reduced their rights to suit the tastes of the wealthy, in contrast to the fairly strong civic pride of the earlier Empire, when the common people felt like they had some rights which the Empire protected.
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Bribing barbarians was always an easy point for rival Roman politicians to nail you on as 'weak' and 'un-Roman', potentially starting a civil war or a coup... which would weaken the Empire and invite other barbarian tribes to try their hand at invasion... which would start more incidents of the same variety and same effects.
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The Roman military was starved of funds at this time, and so when they were needed for an enemy that could no longer be bribed, very often they were not incredibly enthusiastic about being overworked, underequipped, undertrained, and underpaid, sent against an enemy that their own leaders had been paying handsomely for the past few years. It did not help recruitment or personnel retention - even with the Late Roman Army being conscripted for life sentences, on pain of death (in contrast to the earlier, fixed-term volunteer Legions).