The river is notable as Roman law prohibited the Rubicon from being crossed by any Roman Army legion. The river was considered to mark the boundary between the Roman province of Cisalpine Gaul to the north and Italy proper to the south; the law thus protected the republic from internal military threat. When Julius Caesar crossed the Rubicon with his army in 49 BC, supposedly on January 10 of theRoman calendar, to make his way to Rome, he broke that law and made armed conflict inevitable. According to historian Suetonius, Caesar uttered the famous phrase ālea iacta est (“the die is cast”).
Suetonius also described how Caesar was apparently still undecided as he approached the river, and the author gave credit for the actual moment of crossing to a supernatural apparition. The phrase “crossing the Rubicon” has survived to refer to any people committing themselves irrevocably to a risky and revolutionary course of action – similar to the current phrase “passing the point of no return“. It also refers, in limited usage, to its plainer meaning of using military power in a non-receptive homeland.