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The Rubicon (Rubico) is an ancient Latin name for a small river in northern Italy. In Roman times it flowed into the Adriatic Sea between Ariminum and Caesena. The actual modern identity of the water-course is uncertain, it is usually identified as the Pisciatello in its upper reaches and then the Fiumicino to the sea.

The river is notable as Roman law forbade any general from crossing it with a standing army. The river was considered to mark the boundary between the Roman province of Cisalpine Gaul and the Roman heartland, the law thus protected the republic from internal military threat.

When Julius Caesar crossed the river in 49 BCE, supposedly on January 10, in pursuit of Gnaeus Pompeius Magnus he broke that law and made armed conflict inevitable. According to Suetonius he uttered the famous phrase 'the die is cast' [1]. 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 person committing themselves irrevocably to a risky course of action.

[1] Lives of the Caesars 'Divus Julius' sect. 32. Suetonius gives the Latin version, iacta alea est, although according to Plutarch's Parallel Lives, Caesar quoted a line from the playwright Menander: anerriphthô kubos, or 'let the die be cast'. Suetonius' subtly different translation is often misquoted as alea iacta est.