E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
To pass the Rubicon. To adopt some measure from which it is not possible to recede. Thus, when the Austrians, in 1859, passed the Ticino, the act was a declaration of war against Sardinia; and in 1866, when the Italians passed the Adige, it was a declaration of war against Austria. The Rubicon was a small river separating ancient Italy from Cisalpine Gaul (the province allotted to Julius Cæsar). When Cæsar crossed this stream he passed beyond the limits of his own province and became an invader of Italy.