Julius Caesar crosses Rubicon

19th-century illustration, "Caesar crossing the Rubicon." Romans under Julius Caesar crossed the Rubicon River from Gaul into Italy in 49 B.C. Caesar, in breaking Roman law by leading his army back across the river towards Rome, uttered "alea iacta est" ("the die is cast). Thus the expression "crossing the Rubicon" has come to suggest an action that cannot be undone.