Rubicon

A small stream marking the boundary between ancient Gaul and Roman territory. It was made famous (though no one knows for sure today where it is) by Julius Caesar who crossed it with his army for Rome when he felt that his power and future was being threatened by the Roman Senate, and in doing so, sparked a civil war which brought him into position of dictatorship. To 'cross the Rubicon' has, in time, come to mean, metaphorically, making a decision from which there is no turning back.