Nothing in history is inevitable. There are a gazillion different outcomes that could have happened instead of Augustus taking absolute control over the Republic's institutions (which remained in place deep into the 4th century; officially, the Roman "empire" was still the Roman Republic right up to the point Diocletian established the Dominate. Yeah, that kind of changes your outlook, when you think of the Roman empire as a military dictatorship, eh?).
Still, AFAIK (and I'm no expert), the Republican civil wars were the eventual result of centuries of conflict between the patricians and the lower classes over political power, something which eventually split the ruling class and subverted the Republic itself.
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