system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5: can anyone explain "crossing the rubicon?" I've wiki'ed it, so I know it's a river in Italy. Everything else is a bit too dry for me to digest.
gpt
In ancient Rome, generals weren't allowed to cross the Rubicon with their troops - the idea was that the generals would sort of disband their armies before officially entering Italy, making it impossible for a general to try leading his army in a civil war or a coup.  Crossing the Rubicon with your army was punishable by death.

Well, Caesar had big ideas, and he was pretty sure that he could win a Civil War.  But still, crossing the Rubicon was the point of no return - once he and his army crossed it, they would be outlaws, and they'd either have to conquer Rome or die trying, because the alternative was execution.

So "crossing the Rubicon" now means "making a point of no return decision."  Because once you've made that tough call, there's no turning back.