When Julius Caesar was removed from his command by the Roman Senate in 49 BC, he uttered “Alea iacta est” (the die is cast) and crossed the Rubicon river with his troops. This step was naturally illegal under Roman law and Caesar thereby irrevocably committed himself to an act of rebellion. As a result, the phrase reproduced above is generally taken to refer to a step that will set in train an irreversible chain of events. At a later stage, Caesar met his ultimate fate at the hands of Brutus and other conspirators, on the Ides of March.
Some 2,000 years later, David Cameron’s Rubicon moment came on 20 February when, on returning from the negotiation of a new settlement with other EU Member States, he announced a firm date for a referendum on the UK’s continued membership of the EU. Matters proceeded inexorably from there, until David Cameron encountered his own Ides of March on the morning of 24 June through the referendum’s “Out” vote.