Historians inform us, that one of the chief causes of the destruction of the ROMAN state, was the alteration, which CONSTANTINE introduced into the finances, by substituting an universal poll-tax, in lieu of almost all the tithes, customs, and excises, which formerly composed the revenue of the empire. The people, in all the provinces, were so grinded and oppressed by the publicans, that they were glad to take refuge under the conquering arms of the barbarians; whose dominion, as they had fewer necessities and less art, was found preferable to the refined tyranny of the ROMANS.--David Hume "Of Taxes"*
If we look for the causes which first led to the overthrow of the Roman Empire, they will be found to have had their source in the employment of Gothic mercenaries, for from that hour the strength of the Romans began to wane and all the virtue which went from them passed to the Goths. --Machiavelli, The Prince (ch. 13)
It's clear that Hume had Machiavelli''s The Prince open at his desk while writing many of his essays--he does not shy away from explicitly praising and critically engaging with Machiavelli by name. Hume and Machiavelli agree, in fact, that the decline of the Roman Empire was a consequence of bad policy. But they disagree about its root cause: Machiavelli claims that fall was set in motion in virtue of employing foreign mercenaries--from that moment on the Roman rulers were not in control of their own destiny.
From our vantage point, Hume, who is writing in the age of the rising standing army, seems more modern with his focus on the consequences of disastrous tax policy. Constantine's willingness to change customary tax-policy (already a grave mistake in Hume's political understanding) in favor of an unjust tax-regime turns out to be disastrous.** (Margaret Thatcher also learned this the hard way, ignoring the fact that Adam Smith called the poll-tax on freemen "altogether arbitrary or altogether unequal.") In Hume's account the roman rulers lost their authority because the heavily taxed citizens preferred the lighter taxes of the barbarous foreigners. In fact, in claiming this Hume agrees with Machiavelli's general point that a wise ruler should keep taxes low, and not mess with his citizens' property.