What Does the Expression 'Crossing the Rubicon' Mean?

On January 10, 49 BC, Julius Caesar defied the Roman Senate and crossed the Rubicon River. This was the start of the Roman Civil War which ended the Roman Republic. From the article:

"What Does the Expression 'Crossing the Rubicon' Mean?

To cross the Rubicon is a metaphor which means to take an irrevocable step that commits one to a specific course. When Julius Caesar was about to cross the tiny Rubicon River in 49 B.C.E., he quoted from a play by Menander to say "anerriphtho kybos!" or "let the die be cast" in Greek. But what kind of die was Caesar casting and what decision was he making?

Before the Roman Empire
Before Rome was an Empire, it was a Republic. Julius Caesar was a general of an army of the Republic, based in the north of what is now Northern Italy. He expanded the borders of the Republic into modern France, Spain, and Britain, making him a popular leader. His popularity, however, led to tensions with other powerful Roman leaders.

Having successfully led his troops in the north, Julius Caesar became governor of Gaul, part of modern-day France. But his ambitions were not satisfied. He wanted to enter Rome itself at the head of an army. Such as act was forbidden by law.

At the Rubicon
When Julius Caesar led his troops from Gaul in January of 49 B.C.E., he paused on the northern end of a bridge. As he stood, he debated whether or not to cross the Rubicon, a river separating Cisalpine Gaul—the piece of land where Italy joins the mainland and at the time inhabited by Celts—from the Italian peninsula. When he was making this decision, Caesar was contemplating committing a heinous crime.

If Caesar brought his troops from Gaul into Italy, he would be violating his role as a provincial authority and would essentially be declaring himself an enemy of the state and the Senate, fomenting civil war. But if he didn't bring his troops into Italy, Caesar would be forced to relinquish his command and likely be forced into exile, giving up his military glory and ending his political future.

Caesar definitely debated for a while about what to do. He realized how important his decision was, especially since Rome had already undergone a ​civil dispute a few decades earlier. According to Suetonius, Caesar quipped, "Even yet we may drawback, but once cross yon little bridge, and the whole issue is with the sword." Plutarch reports that he spent time with his friends "estimating the great evils of all mankind which would follow their passage of the river and the wide fame of it which they would leave to posterity."

The Die Is Cast
The Roman historian Plutarch reported that at this critical moment of decision Caesar declared in Greek and in a loud voice, "let the die be cast!" and then led his troops across the river. Plutarch renders the phrase in Latin, of course, as "alea iacta est" or "iacta alea est."

A die is simply one of a pair of dice. Even in Roman times, gambling games with dice were popular. Just as it is today, once you've cast (or thrown) the dice, your fate is decided. Even before the dice land, your future has been foretold. "Let the die be cast" itself is an expression meaning roughly "let the game begin," and it comes from a play called Arrhephoros ("the Flute Girl"), a comedy written by the Greek playwright Menander in the 4th century B.C.E. Menander was one of Caesar's favorite dramatists.

When Julius Caesar crossed the Rubicon, he started a five-year Roman civil war. At the war's end, Julius Caesar was declared dictator for life. As dictator, Caesar presided over the end of the Roman Republic and the start of the Roman Empire. Upon Julius Caesar's death, his adopted son Augustus became Rome's first emperor. The Roman Empire started in 31 B.C.E. and lasted until 476 C.E.

Therefore, by crossing the Rubicon into Gaul and starting the war, Caesar threw the dice, not only sealing his own political future but effectively ending the Roman Republic and beginning the Roman Empire."