English Idioms and Phrases: Cross the Rubicon
To 'Cross the Rubicon' means to deliberately go past the 'point of no return' (click link for point of no return meaning) which means that something or someone has, on purpose, gone beyond a point that it is impossible turn back or return to where they started.
"When I quit my job and became a painter, I crossed the Rubicon to a poorer life"
"When I sold my house and became homeless I crossed the Rubicon into an uncertain future"
The Rubicon was a shallow River in Italy near the town of Rimini in eastern Italy (the river has been renamed Fiumicino)
In ancient Rome, generals were forbidden to bring their army into the home states of the Roman republic and the territories of Gaul and Rome were separated by the Rubicon river. If any troops crossed the Rubicon river it was considered an act of treason for which the general would be executed.
Julius Caesar was a general and was seen by the Roman senate as a threat to their control. He was asked to stand down (resign) and disband his army. He was given two choices. Do as the senate asked or cross the river and commit treason which would start a civil war.
He decided to cross the river and start a civil war that led to Julius Caesar becoming emperor of Rome in 49BC.
Apparently when started to cross he used another famous phrase 'the die is cast' (click link) and deliberately went past the 'point of no return' as if he won he would be emperor and if he lost he would die.