What caused the outbreak of the Roman Civil War?

The Roman civil wars ran sporadically over about a hundred years. It began when a Tribune of the Plebs Tiberius Gracchus attempted to restore the public lands filched by the aristocracy from the common land used by the poor small farmers (who also constituted the army). He was murdered, his brother after him, and the political fabric and stability of the Republic was fatally disrupted. This was accentuated by Gaius Marius who, to get the numbers of soldiers necessary to repel the Germanic invasions, enlisted the non-propertied class into the army. After their term, these had no farms to return to, and had to rely on their generals to get them a livelihood. This gave the generals large followings, and they used this to advance their own power. Sulla tried to rebalance things, but these competing generals reversed his settlement after he died, and the competition ended up with the war between Pompey and Caesar. The winner Caesar made himself dictator for life to control affairs, but his assassination ended that stabilisation. Then his heir Octavian (later Augustus) with Marc Antony warred against the killers and won. Octavian and Marc Antony ruled the empire, quarreled and warred, with Antony defeated. Octavian instituted a settlement in which, while claiming to restore the Republic, gave himself reserve powers and control of the legions to avoid competition between ambitious generals. This settlement lasted for a couple of hundred years.