Eli5: Why was it world-changing that Caesar crossed the Rubicon?
In: 1397
You might find [this /r/askhistorians thread](https://np.reddit.com/r/AskHistorians/comments/xmcykv/the_rubicon_river_is_one_of_the_most_famous_in/) interesting in this context. The idea of “Caesar crossing the Rubicon” doesn’t appear until some time after Caesar’s death, and the modern idea of what happened came about nearly 100 years later.
The myth behind it is that at the time, Rome had strict rules in place to prevent military dictatorships (or dictatorships of any kind). One of those rules was that Roman armies weren’t allowed into the main part of Italy without explicit permission. Generally the dictatorships that had happened were the result of someone breaking this rule and bringing their armies into Italy, at which point their opponents would rush in as well. This happened a generation or so before Caesar with Sulla, who was briefly dictator of Rome.
In the myth, the Rubicon (noting that no one today is quite sure where it was) was the boundary between where Caesar was allowed to take his army, and where he wasn’t. By crossing it he was essentially declaring war on Rome and would either win – as he did, becoming dictator for life – or lose and die a traitor.
The reality is probably a bit more complicated; tensions were increasing between Caesar and Pompey (his main political rival, based in Greece while Caesar was based in Gaul). Pompey had been married to Caesar’s daughter, in an attempt to keep peace between them, but she had died. Similarly, the third person holding Rome together (Marcus Licinius Crassus – who had become rich and powerful under Sulla’s dictatorship and was a mentor and patron to Caesar) died at the Battle of Carrhae (in what is now Turkey).
There were negotiations between Caesar, Pompey and the Roman Senate to try to sort things out (supposedly Caesar was willing to give up a lot of his power provided Pompey did as well), but as Pompey had troops around Rome the Roman Government was more afraid of him that Caesar, so weren’t willing to order him to disarm.
It is hard to know for sure what happened – there are conflicting accounts, and the more famous ones weren’t written for decades (and they all have some political biases).
The outcome was that Caesar marched his troops on Rome (either before or after Pompey’s Rome had effectively declared war on him). Pompey was outmanoeuvred and ended up fleeing to his stronghold in Greece (where most of his forces were) along with much of the Senate. This was followed by a year or so of fighting across Spain, Greece and the Roman world between them, with Pompey losing, fleeing to Egypt and being assassinated. The civil war would continue for another couple of years (spreading to Egypt, with Caesar supporting Pharaoh Cleopatra against her rival Ptolemy), eventually settling down in 45 BC, with the last Pompeian forces being defeated, a year before Caesar would be assassinated.
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