cross the Rubicon take an irrevocable step.

☞ The Rubicon was a small river in north-east Italy which in the first century bc marked the boundary of Italy proper with the province of Cisalpine Gaul. By taking his army across the Rubicon into Italy in 49bc, Julius Caesar broke the law forbidding a general to lead an army out of his own province, and so committed himself to war against the Senate and Pompey. See also the die is cast atdie.