I think the decline of the Roman Republic began in the 3rd century B.C. as the city extended its sphere of control beyond the Italian peninsula and particularly with the Punic Wars. In the Italian peninsula these trends devastated the plebeian farmers who had made up the citizen legions of earlier wars, many of whom were forced of their small farms into the city of Rome. That amounted to a huge transfer of wealth from plebeians to the patricians who operated huge latifundiae using slave labor. The displaced plebs in the cities were politically destabilizing in a system designed to favor the patricians. The republic faced problems with populist leaders such as Tiberius Sempronius Gracchus, his younger brother Gaius, and Gaius Marius.
The extension of wars beyond the Italian peninsula also resulted in the transformation of the citizen legions into professional legions that were dependent on and loyal to their wealthy generals. This resulted in the domination of the Roman government by wealthy military men such as Gnaeus Pompeius Magnus, Marcus Licinius Crassus, and Gaius Julius Caesar. That ended up with a Civil War between Pompey and Caesar, only to be followed by the “second” Triumvirate of Marcus Aemilius Lepidus, Marcus Antonius, and Gaius Octavius Thurinus and yet another Civil War that was the final nail in the coffin of the Senatus Populusque Romanus and the Roman Republic.
Also, it should be noted that the limited scope of republican government during this period was simply too small to govern the growing empire of the Republic; it was mostly drawn from the 300 members of the Roman Senate. The lands controlled by the Senate were simply too complex for such a small administration. One of the features of the transformation of Roman government by Caesar Augustus was a vastly increased Roman bureaucracy.
In the earliest days of the Roman Republic, wars were fought in Italy. There was also a “war season”–in the summer. The Roman legions were comprised of male citizens of suitable age divided up according to their financial ability to provide suitable arms and armor. (An elite class that could afford a horse was “equestrian” (sometimes called “knights” in modern histories) that made up cavalry; the equestrian class existed long after they had anything to do with cavalry, which in Imperial Rome was generally non-Roman auxiliaries). Citizen farmers operating small family farms made up the bulk of the legionary light infantry. This worked reasonably well because they could work their farms in planting season, go off to war in the war season leaving the farm to the care of children too young and elders too old for military duty and women. Then they could return after the war season in the fall for the heavy work of harvesting.
But when wars spread far afield from Italy, and particularly with the prolonged warfare of the Punic Wars, this system broke down in two ways. First, it meant that the family farmer/citizen legionary was away from the farm for extended periods–even years at a time. As a result the farm became less productive and the farmer and his family fell in to serious debt. And eventually the farm would be abandoned or seized by creditors. These farms then became latifundiae of wealthy absentee landowners in Rome operated by slaves. We have records of instances of this very early in the republican period, but it became a major problem in the third century B.C. and after.
Second, the early republican army was largely financed directly by those citizen soldiers, be they light infantry, heavy cavalry, senatorial commanders, or consular generals. The wars weren’t financed by “the government.” The “pay” that the Roman army received was any booty they could get if victorious–and that was distributed pretty much by rank by the generals. But the Punic Wars required full time soldiers whose substances and arms were provided by the legion–i.e., the general. That came partly from the general’s wealth but mostly from the spoils of war. But the point was that it came from men like Publius Cornelius Scipio Africanus, the great hero of the last of the Punic Wars, not from the Senatus Populusque Romanus. So the loyalty of the legion was to the general not to the Roman Republic. And that meant that the likes of Gaius Marius, his nephew Gaius Julius Caesar, and his grandnephew Gaius Octavius Thurinus (aka Gaius Julius Ceasar Octavianus and to history as Augustus Caesar) could use the legions they commanded against the Senatus Populusque Romanus. And while Marius and Caesar both commanded legions under authority from the Roman Senate, both Marcus Licinius Crassus and the young Octavianus commanded legions bought and paid for entirely from their own fortunes without any authority from the Roman Senate. (Tthe young Caesar before he held any magistracy, raised a navy to capture the pirates who had captured him and an army to put down a provincial revolt–again out of his own fortune and some funds he borrowed, probably from Crassius).
But one of the side effects of all of this is that the city of Rome came to be populated by large numbers of impoverished citizens who had been dispossessed of their farms and bankrupted by war, not only because of the system of manning the legions, but the depredations of the armies of Hannibal in Italy as well. These citizens could vote in the popular assembly for laws and magistrates–including the consuls. They could be swayed by appeals for reform and outright bribes. And, if necessary, they could be excited to become outright mobs–something done by both the populares and the optimates. The authors of the U.S. Constitution were quite aware of this and were concerned to avoid this because they had read their Roman history and were aware that it would lead to tyranny.
Students of history frequently seek cause and effect relationships in the past—as I have done here. The tendency is among those mentioned in David Hackett Fischer’s Historians’ Fallacies: Toward a Logic of Historical Thought, especially chapter VI, “Fallacies of Causation.” (See also, Histories and Fallacies: Problems Faced in the Writing of History by Carl R. Trueman.)
The temptation is to draw questionable parallels between history and current conditions in a predictive ways. An example of this is H. J. Haskell’s The New Deal in Old Rome: How Government in the Ancient World Tried to Deal with Modern Problems. A more recent example would be Are We Rome?: The Fall of an Empire and the Fate of America by Cullen Murphy. I may look at this later. Murphy focuses on Imperial Rome in the second century A.D., but I think the decline and fall of the Roman Republic is perhaps more apt. I our Founding Fathers wished to emulate anything it was republican Rome, not imperial Rome.
Historical events are always unique and unrepeatable, whatever similarities exist between them. While there may be similarities between an event in the past and its presumed causes, there are always significant differences. Among those, it might be noted, is simply the later knowledge of the earlier events and the causal relationships assumed which can later the more modern event. We may, for example, note the extreme disparities of wealth and power in ancient Rome, 18th century France and 19th century Russia and perhaps rightly conclude that they contributed to the “decline and fall” of the respective regimes. We may further note similar disparities in the United States in the 21st century. Whether or not the modern disparity will lead to the ultimate decline and fall of the United States depends in part on how we apply our knowledge of the past to the issues of today.