What does ‘crossing the Rubicon’ mean?

The Rubicon is a shallow river, about 80 km long, in northeast Italy. The idiom ‘crossing the Rubicon’ means to pass a point of no return, and refers to Julius Caesar’s crossing of the river in 49 BC, which was considered an act of war, as the course of the river has frequently changed. It is a point beyond which someone, or a group, must continue on their current course of action, either because turning back is physically impossible or prohibitively expensive or dangerous. The idiom is also used when the distance or effort required to get back is greater than the remainder of the journey as yet undertaken.