(Apologies to the Mods if this one is a duplicate, I couldn't find anything from reddit search or google)
Specifically, I'm speaking with regards to the Civil War involving Caesar and Pompey.
I've always heard that Caesar's pursuit of offices and attempts to keep his armies to avoid being prosecuted eventually led to his clashes with the Roman Senate, which in turn led to him starting a Civil War when the Roman Senate tried to disarm him.
However, I've recently read on this subreddit that the Caesar Prosecution Theory (for lack of a better term) has relatively little actual backing and that he was never really under threat from the Senate. From my point of view, this undermines the idea that he started the Civil War to avoid prosecution by the Roman Senate (it also makes me wonder about the motivations of the people portraying Caesar as a victim there, but that's another matter, I think), but it also now means I have no idea why else it started.
So why did the Roman Civil War start?
So there was no one single Roman Civil War but a series of them between 91 BCE and 31 BCE.
The Late Republic period was characterised by extended period of political instability and social unrest which came to be known as Crisis of the Roman Republic.
Its hard to specifically date the beginning of the crisis. The Republic had been showing signs of political instability and social unrest for decades. From the mid 2nd century BCE Roman politics became increasingly factionalised into two opposing groups - the optimates, representing the senate and the aristocracy, and the populares, representing the plebeians and urban poor.
The major collapse of Roman political order is generally considered to coincide with the tribuneships of Tiberius and Gaius Gracchus between 134 BCE and 123 BCE.
These brothers wished to enact land reform legislation which would transfer land from wealthy aristocrats to poorer citizens and retired soldiers. Recognising that he would never be able to get this legislation past the optimates controlled Senate, Tiberius took the unprecedented step of enacting an obscure law which allowed him to bypass the Senate and instead bring his reforms to the populares controlled plebeian assembly. This shocked the Senate and they did everything in their power to obstruct the practical enactment of Tiberius' new laws by refusing to finance their implementation. Faced with this new obstacle Tiberius took another unprecedented step and lobbied to use a trust fund left by Attalus III of Pergamum to the Roman people, directly challenging the Senate's traditional monopoly over international relations and state finances. The Senate continued opposition and as the end of his term approached Tiberius took one final unprecedented step and sought re-election to complete his reforms. This went against the long held political ideal of strict term limits and was the last step for the Senate. A group of Optimates caught Tiberius and 300 of his supporters in the forum and beat them to death. Tiberius was decapitated and thrown in the Tiber, denying him the chance of a proper burial.
Ten years later his brother Gaius would attempt to use the same methods to pass even more revolutionary legislation, and would meet the same fate as his brother.
Tiberius had singlehandedly shown the major structural flaws in the Roman constitution and how they could be used to force legal change outside recognised traditional procedures. The result was a complete collapse of the traditional political procedure and its replacement with gangs, violence, assassinations, riots and radical factionalism as methods of establishing political power. It was clear that the Republican constitution was broken and the only way to return Rome to political order was by force, and thus began the civil wars.