To do something that inevitably commits one to follow a certain course of action.
Also,to do something which will have very important results, which cannot be changed later.
Also,to make a decision that cannot be changed later.
When I quit editing and decided to be a writer, I had crossed the Rubicon to an uncertain future.
The crossing of the River Rubicon by Julius Caesar inevitably involved him in a war with the senate in 49 B.C.
Jane crossed the Rubicon by signing the contract.
Find another job before you cross the Rubicon and resign from this one.
International pressure may be able to prevent the country crossing the Rubicon to authoritarian rule.