To pass the Rubicon. To adopt some measure from which it is not possible to recede. Thus, when the Austrains, 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 Caesar). When Caesar crossed this stream he passed beyond the limits of his own province and became an invader of Italy.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
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