Why the question keeps being asked
The standard answer is wrong. The Roman Republic did not fall to Caesar; it did not fall to Pompey; it did not fall at Pharsalus or at the Rubicon. Those moments are the visible ones. The Republic that ended in the years between 49 BCE and the Augustan settlement of 27 had been failing as a kind of polity for much longer. The ancient sources understood this. So has most of the serious modern historiography. The argument is about which of the structural conditions was load-bearing — and the argument is not over.
The structural inheritance
By the time Caesar was old enough to hold his first magistracy, the institutional habits that had held the Republic together for four centuries had already been broken — not all at once, not by anyone who intended to break them, but cumulatively. Three layers of unwinding sit underneath the late-Republican careers.
The first is military. The Marian reforms — capite censi recruitment, post-service land settlement secured by the general — meant that by the second century BCE Roman legions were no longer instruments of the senatorial state in the older sense. Their long-term security depended on their commander, not on the city. The Bellum Iugurthinum shows the conditions in which this transition happened; within twenty years Sulla had drawn the obvious conclusion and marched on Rome.
The second is institutional. The conventions of the late Republic allowed extraordinary commands — first to Marius (seven consulships), then to Pompey (the lex Gabinia, the lex Manilia, the sole consulship of 52), and finally to Caesar (nine years in Gaul). Each was treated as a one-time exception necessitated by crisis. The cumulative effect was that by 50 BCE there was no meaningful precedent for refusing such a command, and the idea that the senate was the final political authority had become a constitutional fiction.
The third is moral, in the Roman sense of the word. The Bellum Catilinae dates this to 146 BCE and the destruction of Carthage — the loss of the metus hostilis, the external check on ambition. Whatever one thinks of Sallust's specific date, the diagnosis is recognisable: a political class that had once accepted the mos maiorum as a real constraint had stopped doing so by the generation of Catilina, and the institutions that depended on that acceptance — the senatorial debate, the consular election, the provocatio — became, in the language of Tacitus's later analysis, forms whose substance had drained away.
The careers as expressions, not causes
Read against this background, the late-Republican careers look less like causes and more like the forms the structural unwinding took. Pompey's career, with its unprecedented commands and its insistence on private military reach into politics; Crassus's, with its acquisitive politics and its destruction at Carrhae; Caesar's, with its decade in Gaul and its march back; Cato's, with its refusal to compromise on principles that the institutions could no longer enforce — each is intelligible only against the structure each had inherited. Cicero's De Re Publica, written in 54–51 BCE, is in some way his attempt to recover, as theory, the constitutional substance the practice had stopped supplying.
What we lose if we collapse the question
There is an old tradition that treats the fall of the Republic as a moral story about Caesar's ambition. There is a counter-tradition, running from parts of Tacitus through the German historians of the nineteenth century, that treats it as a structural story about an empire that had outgrown its political form. The first is too small; the second is too large. The careful reading is that the structural conditions made the careers possible and the careers used them ruthlessly. Neither without the other is enough to explain what happened.
Why the question still matters
The Roman case is the longest sustained ancient study of how a republican order is lost. It is not a recipe. The conditions it identifies — military instruments whose loyalty depends on their commander, extraordinary commissions that become normal, political class indifference to its own constitutional fictions — describe a shape of decline, not a destiny. The European republican tradition from the Florentine humanists through the American founders read this material seriously because it raised questions every later republic has had to answer in some form. The questions have not become any smaller for the long time the tradition has lived with them.