To start off with, we need to understand why Machiavelli viewed Roman history the way that he did and what he was trying to do with it. Machiavelli, like historians for the previous 1500+ years and, perhaps to a somewhat lesser extent, the next 500 was absolutely fascinated by the rise of Rome - how did a small city in the middle of Italy, towards the fringes of the civilised world, once ruled by Etruscan kings and endowed with a modest hinterland inhabited by poor yet hardy warrior-peasants, come to be mistress of the Mediterranean, an achievement that no one had ever managed before? Machiavelli, like the ancient historians Polybius (a Greek general captured in the Punic wars), Sallust (a late Republican Roman historian) and Livy (writing in the reign of Augustus) before him, whom he was completely versed in and were his main primary sources for ancient history, saw three main things as being key to Rome's success. The first being the nature of the Republican Roman constitution from the overthrow of the last kings in 509 BC down to the reforms of the Gracchi in 133 BC, which Machiavelli, like Polybius, saw as the perfect balance of the three classical forms of government outlined by Aristotle - monarchy, aristocracy and democracy - that effectively cancelled out their flaws and prevented them from becoming their perversions (tyranny, oligarchy and anarchy). The second was a certain measure of good luck. The third, and most important, was the civic virtue of the ancient Romans - histories of Rome like Livy's Ab Urbe Condita were full of examples of heroic Romans from the first four centuries of the Republic's existence who selflessly served the state in politics, administration and warfare, as opposed to seeking fame, enrichment or lasting power, and were willing to make sacrifices or do acts generally perceived to be dishonourable or immoral (the quintessential example being Brutus, the founder of the Republic, killing his traitorous sons) for the collective good of the Roman people. This civic virtue was above all manifested in the republican institutions, designed to prevent any individual from becoming too powerful, and in Rome's citizen-army (similar to those of Sparta and Athens, but more generous in admitting people not exclusively from Rome itself), which created a direct link between service to the state in war and being an active citizen.

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Machiavelli essentially perceived the transition from Republic to Empire and the Imperial period in light of this, both because that was how the Roman historians he worked with saw it but perhaps also to give his historical narrative unity and elegance - renaissance humanists saw history as a branch of literature, but one that, unlike poetry, dealt with things that were true rather than things that were merely plausible. He argued that the corrupting influence of Rome's phenomenal wealth and success and the undermining of republican institutions, culminating in the creation of an autocratic emperor under Augustus, after the end of the Punic wars sapped away at Rome's civic virtue. This declining civic virtue, so he believed, led to the Romans no longer wanting to perform civic service in war and instead making humiliating deals with its neighbours, like the Persians, and hiring foreign mercenaries from the Germanic tribes which made it vulnerable. He also saw the rise of Christianity as further sapping away at the Roman civic spirit by promoting meekness, humility and an absolutist conception of right and wrong action, the three things he saw as ruinous to successful statecraft and warfare. This view of Roman history that Machiavelli formulated, contrasting the inexorable rise of poor, hardy, militaristic, civic-spirited, pagan Republican Rome with affluent, soft, degenerate, servile, ultimately becoming Christian Imperial Rome, would come to be hugely influential. It would undoubtedly influence Baron de Montesquieu and Edward Gibbon's classic eighteenth century enlightenment era formulations of the fall of Rome, which subsequent generations of historians have essentially been debating ever since, and in the world of philosophy and politics would have some influence on Nietzsche, Mussolini and Hitler.

Yet for all its influence, Machiavelli's approach to Roman history is based on an approach and agenda profoundly different to those of modern academic historians. Machiavelli, like other renaissance humanists, believed that the purpose of history was not to understand the past for its own sake, but to find moral and political lessons for the present day. In the Italy of his own day, gripped in the midst of the great renaissance European superpower conflict known as the Italian wars (1494 - 1559), Machiavelli saw the old city republics of high and late medieval Italy being humiliated or utterly crushed due to, as he saw it, corruption and decadence, the growth of tyranny and oligarchy, over-reliance on untrustworthy foreign mercenaries (the condottieri) willing to pledge their services to the highest bidder and the meddling of the papacy and foreign powers (France and the Hapsburg Empire in particular, whom he called "the barbarians"). Comparing that to the earlier history of the Italian states in the twelfth to mid-fifteenth centuries, that had successfully fought for their independence, expanded their hinterlands and preserved civic liberty and republican governance, Machiavelli would have identified parallels between that and Roman history. And so through his works - not just "the Prince" but also "Discourses on Livy" (his most important work in my view, and the truest expression of his political and ethical views), "The Art of War" and "The History of Florence", he wanted to impart these lessons in hope that his contemporaries could rescue Italy from its current situation. "The Prince" and "The Discourses on Livy" definitely stand as landmarks in renaissance history-writing and political theory in the eyes of modern historians, but their arguments would definitely not meet modern standards of historical scholarship. For more on Machiavelli's approach to history writing, I'd highly recommend you read "Machiavelli and Guicciardini" by Felix Gilbert.

u/reproachableknight gave a fantastic response in regard to Machiavelli’s views and how they were shaped by the historicism of Renaissance Italy. I would like to supplement this with some perspective on the time period that Machiavelli was addressing and how modern views differ may from his own.

We must address the idea of the “collapse of the Roman Empire.” This notion is heavily ingrained into western conceptualization of history with the traditional narrative being that the Roman Empire fell on September 4, 476 A.D., the day that Odoacer, leader of the foederati - a consortium of non-Roman tribes who were in alliance with the Roman Empire- deposed Emperor Romulus Augustulus. The purported reason for this is that Augustulus’ father, Orestes, had reneged on his deal with the foederati that they would be entitled to settle in 1/3rd of the lands of Italy if they helped to enact the coup against emperor Julius Nepos that made Augustulus the new emperor.

But is it that really the case? When the Roman Empire was divided between East and West, the system established was that each emperor had to recognize the other, and the Eastern Emperor Zeno never recognized Augustulus as emperor. Of course, Zeno was himself deposed in 475, but regained power in 476. So then did the empire end when Nepos was deposed in 475? Well, when Odoacer became King of Italy, he entered into negotiations and relations with Eastern Emperor Zeno and gave at least de jure acknowledgment of Nepos being the restored Western Emperor and the Roman Senate did retain authority over Roman citizens.

When Nepos died in 480, Zeno declared the Western Empire was no more, but what did this mean? Traditionally, this is seen as an overdue acknowledgment that the empire held no more power in the West. A number of modern historians, however, see this as Zeno reuniting the empire, with administrative duties in Italy being shared between the Senate and the King/Patrician of Italy, who had different roles as to the Roman citizens and the non-Romans in Italy. And as Odoacer continued to exercise de facto independence, in 490, Zeno convinced Theodoric Amal, a Goth who grew up as a hostage in the court of emperor Leo I and held official positions in the Eastern Empire, to conquer Italy from Odoacer. Now while Theodoric was magister militum and at one point a consul he also alternated between friendly and hostile relations with Zeno. He was very much Roman in some ways, and very much non-Roman in others. He waged war against Odoacer until they came to a truce and agreed to rule Italy jointly. Then Theodoric invited Odoacer to a dinner, and had him killed while under his roof. He went on to rule Italy until 526 as King of Italy and King of the Ostrogoths. He was succeeded by his grandson, who was often caught between his mother, who wanted him to strengthen the bond between the kingdom and the emperor as a Roman ruler, and members of the gothic court who wanted him to be a goth and prioritize gothic interests. He died young and all hell broke loose after that, leading to the Gothic Wars.

It is also worth noting that Zeno himself was often unpopular as he was of barbarian origin and only ascended to the throne because he married Leo I’s daughter, and their son, child emperor Leo II, died young. From 484-484, Zeno was faced with revolts carried out by his own generals, including Leontius, who claimed the throne, and Illus, who had helped Basilicus depose Zeno in 475, then changed sides to ally with Zeno, served him for a time, but later allied with Leontius.

The reason I say all of this is to give one slice of a greater reality: that the politics of Rome in late antiquity were messy, constantly in flux, with frequent revolts, coups, dealings, alliances, and hostilities between multiple parties. When examining the history of the empire and the republic that preceded it, we find that this is itself somewhat par for the course, although there were periods that were more stable than others. In fact, Edward Gibbon largely dismissed political infighting as a factor in Rome’s decline because it was so prevalent throughout.

In light of this, we are faced with the question of how to define the entity of the Roman Empire, more specifically the Empire in the West. Is it the political infrastructure? How do we apply this to the reality of frequent changes and shifts in power? Do we judge Roman authority by the central authority or by local entities? How do we weigh de jure authority against de facto power? Should we instead look at the issue in terms of Roman civilization instead of the political entity? How do we define this? How do we define Roman identity? How do we account for regionalism? How do we view the introduction of Germanic elements into Roman society? What role does religion play in all of this?

Today, historians have different views as to whether the empire collapsed or transformed, what led to these scenarios, and were they primarily internal or external developments. Those who say that the empire collapsed have identified over 200 potential factors that contributed to what can be considered either a catastrophic collapse or gradual decay. Those who favor transformation models look at many of those factors and others, and instead see the roots of medieval European power structures developing within the foundations of Roman society long before the migration period, such as the increasing role of the Church in daily life and the development of the villa system into feudalism. In this view, all of the messy politics and infighting between figures such as Augustulus and Odoacer and Zeno and Theodoric are mere reflections of a changing political environment rather than its driving force.

I have found in my own studies that when faced with these different models and factors, those questions I posed about defining the Roman Empire become somewhat determinative of how an individual might interpret such a vast and complex subject matter. This is true not only of today’s historians, but also of ancient ones such as Augustine and Zosimus. Even when examining today’s political landscape, we see people come to wildly different conclusions based on how they define their culture, their society, and its threats. Are threats largely internal or external? How important is a shared culture in comparison to diversity?

People who have stronger nationalist ideas such as Machiavelli or Peter Heather tend to favor models that show a clear divide between one group of peoples and the other such as between Roman civilization and the Germanic medievalism that succeeded it. Notably, Machiavelli, as the forebear of Italian nationalism, held the Romans in higher esteem whilst Heather praises the Germanic tribespeople as having a proclivity for freedom (he has been criticized for approaching if not crossing the line into promoting ethnic superiority). People with this sort of view tend to favor invasion-and-collapse models. Others, such as Peter Brown and Jonathan Arnold, do not see such a clear ethnic divide, rather they view the world of Late Antiquity on more of a cultural and political continuum. Their views are more in line with the ideas of transformation, with the influence of Germanic peoples invading, but also integrating and synthesizing with the already diverse Roman culture. Some think that this view minimizes the role of military hostilities, but as I said before, those were largely present throughout the empire’s existence.