cross the Rubicon

1. To make an irrevocable decision; it comes from the name of the river Julius Caesar crossed with his army, thereby starting a civil war in Rome . ( See Rubicon .)

Idioms and Phrases

Irrevocably commit to a course of action, make a fateful and final decision. For example, Once he submitted his resignation, he had crossed the Rubicon . This phrase alludes to Julius Caesar's crossing the Rubicon River (between Italy and Gaul) in 49 b.c. , thereby starting a war against Pompey and the Roman Senate. Recounted in Plutarch's Lives: Julius Caesar (c. a.d. 110), the crossing gave rise to the figurative English usage by the early 1600s.