"to cross the Rubicon"?
In common language it means to cross the point of no return beyond which there is no turning back.
It means to make an ireversible and irrevocable move with immense implications and ramifications leading to profound changes.
It refers to Julius Caesar crossing the Rubicon, a small river which was the boundary between Italy and Cisapline Gaul (Gaul on the Italian side of the Alps).
Julius Caesar commanded the Roman legions in Gaul and ancient Roman law prohibits a general to lead his troops across the Rubicon River into Italy. The purpose of the law being to prevent the legions from acting against the Senate.
The Senate had become fearful of Caesar's power and suspicious of him and demanded that he disband his legions. Instead, Caesar led one his legions, the Legio XIII Gemina, across the Rubicon on 10 January 49 BC -- in effect, declaring war on Rome itself.
It started a civil war in the Roman Republic with Caesar leading his legions and supporters, the faction known as the Populares against the legions of Pompey who supported the conservative faction of the Senate known as the Optimates.
The end of the civil war resulted ultimately in Rome remaining a republic in name but becoming an empire in fact under his successor, his nephew Octavian who became Rome's first emperor -- Augustus Caesar.
Author of "Taierzhuang 1938 – Stalingrad 1942" (ISBN 9780983843597)
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