Cross the Rubicon
To "Cross the Rubicon" means that a decision has been made or an event has happened that cannot be changed.

In 49 BC, Julius Caesar made the fateful decision to lead his army across the Rubicon River into the northernmost of Rome's territory, thus committing to a civil war against the Roman Senate. Just before he crossed this river, Caesar is reported to have said "The Die is Cast" which phrase survives today with the same meaning as "cross the Rubicon".

Use Example - We need to seriously think about whether we want to talk to the boss about this; it's like crossing the Rubicon.

Source Tags : Julius Caesar