Why did Julius Caesar say, 'The die is cast', when he declared war on Rome?

Julius Caesar's rise to power did not seem to be very likely in the early years of his life. His first real political elevation began with his election as pontifex maximus in 63 BC, which granted him significant religious influence in Rome.

Two years later, in 61 BC, he became governor of Hispania Ulterior. As governor, he demonstrated his military capabilities by subduing local tribes and securing his province.  

However, by the end of his tenure in 60 BC, Caesar returned to Rome, seeking the ultimate position in the Roman government: the consulship.

However, Caesar knew that his chances of winning the election was almost impossible, as he did not have the reputation nor the wealth to convince enough Romans to vote for him. 

To solve this problem, Caesar orchestrated one of the most illegal and secretive agreements ever formed in Roman political history.

Known as the First Triumvirate, Caesar oversaw a three-way promise between himself, Pompey the Great, and Crassus.

Using their combined influence, wealth, and social prestige, they worked together in the shadows to achieve their own political aims.

For Caesar, this meant that the other two men helped him secure the consulship in 59 BC. 

However, during his consulship, Caesar continued to use the secret arrangement to force through numerous reforms to benefit his allies.

As a result, he gained considerable support from the plebeians. By the end of his year as consul, the details of the First Triumvirate had become public knowledge and the Senate sought to bring Caesar to trial.

In order to protect himself, Caesar needed a new political position. This would protect him since any person in a government role was immune from legal action as long as they were in power.

So, while he was consul, the Senate could do nothing. 

To extend his immunity, Caesar used his influence to secure a new position for the following year: 58 BC. He received the proconsul of Gaul, which provided him with a 5-year position as a military commander.

So, over the next decade (since he was able to get an extra 5-year extension), he conquered vast territories and amassed wealth and prestige in Gaul.

However, his crucial relationship with Pompey and Crassus grew strained. By 53 BC, the death of Crassus left just Pompey and Caesar. 

Unfortunately for Caesar, in 52 BC Pompey had switched his allegiances and was now working with the Senate. The Senate was wary of Caesar's ambitions.

In January 49 BC, the Senate issued an ultimatum to Caesar: disband his army and return to Rome as a private citizen to face judgement for his actions or be declared an enemy of the state.

Caesar, however, knew returning without his army would leave him vulnerable to his enemies. Thankfully, successes in Gaul had made him popular among the people and his soldiers, which meant he thought he could rely upon the people for help.

So, he openly defied the Senate's order and declared himself as a defender of the people's rights against the corrupt Senate. 

On the 10th of January 49 BC, Julius Caesar and his legions had marched from Gaul and into northern Italy. It was here that they reached the banks of the Rubicon.

This shallow river marked the official boundary between the Roman province of Cisalpine Gaul and Italy proper. By law, Caesar could cross it with an army without facing severe consequences.

With him was his most loyal and feared 13th Legion, which numbered about 5,000 seasoned soldiers. 

As he stood at its edge, Caesar knew the gravity of his choice, so weighed his options carefully. To return to Rome without his army meant facing prosecution, loss of political power, and possible execution by his enemies.

His dignitas, or personal honor, was at stake. Furthermore, his soldiers' unwavering loyalty compelled him to act decisively.

With Pompey and the Senate's forces numbering around 45,000, the odds seemed daunting.   

With these considerations in mind, Caesar turned to his men and uttered the fateful Latin words, Alea iacta est. It was a variation of an ancient Greek phrase Anerriphtho kybos used by the playwright Menander, which referred to the rolling of dice in a game and not knowing what the result would be.

Ultimately, Caesar was declaring his decision to be an irreversible gamble. Therefore, knowing full well that crossing the Rubicon with his army would initiate a civil war, Caesar walked across the river and into Italy.

His soldiers, loyal and battle-hardened, also knew the risks they faced. Yet, they followed him across the Rubicon without hesitation. 

Caesar moved swiftly south, leaving his enemies little time to react. Upon learning of Caesar's advance, Pompey and the Senate fled Rome in panic.

They retreated to the south, gathering forces to oppose Caesar's army and crossed over to Greece to, hopefully, strengthen their position.

Within days, Caesar's forces occupied key locations, including Ariminum and Pisaurum, seizing control over central Italy. 

The ensuing conflict saw battles across the Roman world, from Spain to Greece. Each skirmish and siege added to the chaos and instability.

In 48 BC, Caesar pursued Pompey to Greece, leading to the decisive Battle of Pharsalus. This battle, fought on August 9, 48 BC, saw Caesar's 22,000 men facing Pompey's 45,000.

Despite being outnumbered, Caesar's strategic brilliance led to a crushing victory. His forces routed Pompey's army, causing significant losses.

Pompey fled to Egypt, seeking refuge. However, upon his arrival, he was assassinated by agents of Pharaoh Ptolemy XIII. 

After Pharsalus, Caesar continued to consolidate his power. He dealt with resistance in North Africa and Spain, securing crucial victories.

At the Battle of Thapsus in 46 BC, Caesar defeated the remaining senatorial forces led by Metellus Scipio and Cato the Younger.

Following this, the Battle of Munda in 45 BC marked the final blow to the senatorial opposition. Caesar's forces decisively defeated the army led by Pompey's sons, ending significant resistance.

Throughout the civil war, Caesar demonstrated remarkable leadership and tactical prowess. His ability to inspire loyalty among his troops proved invaluable.

By the end of the conflict, Caesar stood unchallenged as the master of Rome. His actions and decisions during the civil war fundamentally altered the course of Roman history, leading to the end of the Republic and the rise of the Empire.