To take an irrevocable step, to burn one’s bridges, to go beyond the point of no return.
The Rubicon was a smaller river, which formed the border between ancient Italy and Cisalpine Gaul, the province allocated to Julius Caesar (100-44 BC). When Caesar crossed this stream in 49 BC, he went beyond the limits of his own province and thus became an invader in Italy, making the outbreak of war between Pompey (106-48) and the Senate inevitable.
“The Rubicon” is now often used alone as a description of “the point of no return”.
Taken from Judy Parkinson, “Spilling the Beans on the Cat’s Pyjamas”, Michael O’Mara Books Limited.