The phrase "crossing the Rubicon" is an idiom meaning "passing the point of no return". Its meaning comes from the crossing of the Rubicon river by Julius Caesar in January 49 BC at the head of the 13
An idealized 1849 illustration of the crossing by Jacob Abbott (History of Julius Caesar). Modern historians, such as Mary Beard, note that Caesar likely crossed at night in a carriage, rather than on horseback as depicted here.