What is the meaning of "crossing the Rubicon"?

"Crossing the Rubicon" is a common metaphor for the making of a significant choice that a person cannot change. It can also refer to the passing of a point of no return.

The phrase "crossing the Rubicon" stems from a historical event. English speakers began to use it as an expression in the early 17th century. In 49 B.C., Julius Caesar brought his army across the Rubicon Rubicon River from Gaul into Italy. Gnaeus Pompey Magnus, or Pompey the Great, was the leader of the Late Roman Republic and did not allow generals to cross borders with troops. Caesar's action resulted in a civil war and the formation of the Roman Empire.