Definition & Meaning of "the die [is] cast"

What is the origin of the idiom "the die is cast" and when to use it?

The idiom "the die is cast" finds its origin in ancient Rome and is attributed to Julius Caesar. According to historical accounts, in 49 BCE, Julius Caesar led his troops across the Rubicon River, a boundary that marked the limit of military authority in Rome. This act defied the Senate's orders and effectively started a civil war. As Caesar crossed the river, he reportedly exclaimed "alea iacta est" in Latin, which translates to "the die is cast." The phrase refers to the throwing of dice in gambling, where once the dice are cast, the outcome becomes fixed and irreversible. This phrase can be used in personal, professional, or historical contexts to convey the idea that a course of events has been set in motion, and there is no possibility of changing or undoing the outcome.