“Crossing the Rubicon”
means to pass a point of no return.
The phrase refers to Caesar's 49 BC crossing of a minor river in Northern Italy
in direct violation of the orders of the leaders in Rome, who feared his power.
A civil war followed, in which Caesar emerged as ruler of Rome.
Caesar is supposed to have said, “The die is cast”
(referring to a roll of dice), as he crossed the river.