Cross the Rubicon
Etymology
In 49 BCE Julius Caesar led his army across the Rubicon, the river marking the boundary of his province, into Italy — an act of open rebellion against Rome, reportedly with the words "alea iacta est" ("the die is cast"). There was no legal or practical way to undo it, so it triggered civil war. The phrase now marks any committing, unrepeatable step.
verb · /KROS thuh ROO-bih-kon/
To take an irreversible step that commits you to a course with no turning back.
What it names
For the exact moment a decision passes the point of no return and cannot be walked back.
Example
By publicly naming the whistleblower, the agency crossed the Rubicon.
Related idea
A watershed is a moment after which things are different; crossing the Rubicon is the deliberate, irreversible act of committing — a turning point versus a point of no return.