Check out the first post in this series for the background to Caesar's wider struggles.
I don't really want to sit here and recount the entire Gallic Wars, because that would be tiresome to say the least. Caesar himself wrote an extensive memoir on his time in Gaul. I admittedly have not read it. Historians agree that it is Roman propaganda as far as battle statistics, but that the general course of events is accurate. We're here to talk about the many bad things Caesar did.
Most of Caesar's conquests cover modern France. His wars lasted for nearly a decade, although they culminated in 52 BCE at Alesia. Here Caesar pulled his famous 'donut wall' tactic, sealing his army in between the city and relief forces. This was the victorious end to Caesar's war on Vercingetorix, 'King of the Gauls' and fighting after that was markedly less intense. Vercingetorix, had been raised as a captive amongst the Romans and so knew their art of war, making him Caesar's most dangerous opponent to date. Caesar ultimately defeated his foe and the Roman Republic now included much of western Europe.
The problem is that Caesar started his wars under false pretenses, essentially conjuring them for the sake of conquest. His casus belli ranged from migrations through allied lands to outright attacks and finger-pointing. He went around conquering territory for Rome with absolutely no right by law or custom, nor permission by his own superiors, to do this.
Caesar was also accused of war crimes for the way he treated the conquered lands and peoples. Romans enslaved their enemies but Caesar was seen as, to use modern vernacular, a dick. His legionaries loved him, the plebeians believed in him as a champion of their cause, but the optimates and other Senators despised him. While Caesar held command in Gaul he was above the law, anything he did could not legally be considered a crime until he surrendered command.
This is what set Pompey and Caesar apart. This is what caused Caesar so much fear and anxiety. He knew his enemies would prosecute him for war crimes as soon as he forfeited command upon returning to Rome, even to stand for election. Instead he illegally retained his command for years, waging illegal wars, committing war crimes, a rogue general detested by the Senate.
He knew he would be punished severely if he ever became a civilian again. It just wasn't an option, what his enemies were talking about approached treason. Cries in Rome grew louder for the recall of Caesar to answer for his actions. He was still too popular in the Senate, which would play an important part in his story later.
With all of this in mind, Gaius Julius Caesar sat down one fateful night and considered his options. The next day he ceremonially crossed the River Rubicon, famously uttering that, "the die is cast." He had made his choice.
He was followed by the men of the Legio XIII Gemina, veterans of his wars in Gaul. Now was the time for action and Caesar was eager to take it.
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