A critical decision point beyond which there's no turning back; a boundary that, once crossed, commits you to a course of action.
From the Rubicon River in northern Italy, which Julius Caesar crossed with his army in 49 BC, defying the Roman Senate's order—the act that started a civil war and is why we say 'crossing the Rubicon.'
When Caesar crossed the Rubicon, he reportedly said 'Alea iacta est' ('the die is cast')—that single river crossing literally determined the fate of the Roman Republic and changed world history, making one geographical boundary the most consequential border in Western civilization.
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