Cross the Rubicon

In 49 BC, Julius Caesar led his legion across the Rubicon, a small river in northern Italy marking the boundary between his province and Italy proper. Roman law forbade a general from bringing troops into Italy itself, so the crossing was an open act of civil war against the Senate. The historian Suetonius, writing in The Twelve Caesars around 121 AD, records Caesar uttering "alea iacta est" (the die is cast) at the moment he crossed, a line itself borrowed from an earlier Greek playwright. The idiom in English, using the crossing as shorthand for any decisive, irrevocable move, developed centuries later, but it rests on this clearly documented classical account rather than any later embellishment.