The conspiracy against Caesar likely festered for a long time, but it only began to take on a legitimate threat in the early part of 44 BC. In a Senate famous for political wrangling and back-stabbing, a conspiracy of such grave magnitude could not have been kept secret for long. The urgency of the plot was not only the fact that Caesar was expected to leave Rome for Parthia on March 18, but that Caesar was expected to be named King of all the Roman provinces outside of Italy. The apparent leader of the plot was Gaius Cassius Longinus, the Praetor Peregrinus in 44 BC. He already held a grudge against the dictator for perceived political slights, mainly in not backing him for positions of higher authority. In fact, Caesar seems to have had suspicions of Cassius already, but apparently did nothing to counteract it. Caesar often made comments regarding his own death by suggesting he'd prefer to die one death than die countless times as a coward. On a similar note, Cicero once quoted Caesar as saying, "I have lived long enough both in years and in accomplishment."
Whether or not Caesar had any idea of the conspiracy growing against him is certainly debatable but he was well aware of the ramifications of such an act. Suetonius quoted him as saying "It is more important for Rome than for myself that I should survive. I have long been sated with power and glory; but, should anything happen to me, Rome will enjoy no peace. A new Civil War will break out under far worse conditions than the last." Though this prophetic statement would definitely turn out to be the truth, the conspiracy continued to grow. Some 60 senators (of the 900 member body, most of whom were loyal to Caesar) would align themselves in the favor of the Republican ideal, including Caesar's close friend Decimus Junius Brutus Albinus. One notable omission from the conspiracy was the great orator Cicero. Despite his support of Pompey and the Republican side in the civil war, it was feared that Cicero didn't have the fortitude to carry out the task, or that he wouldn't be able to keep his famous mouth closed.
Cicero, however, was not the key to the plot's success. The key ingredient was the inclusion of Caesar's friend, Marcus Junius Brutus. He had served with Caesar in Gaul before taking sides with Pompey in the civil war, and was later pardoned for his 'transgression'. It was rumored, due to Caesar's well known affair with Brutus' mother Servilia, that Brutus could've been Caesar's son. While this is completely unlikely, there is no question that Caesar maintained a relationship of fondness for Brutus, and had supported him for many offices of importance. Brutus was the key for one simple reason. His supposed ancestor, Lucius Junius Brutus, was the man who took the lead in expelling the Etruscan kings centuries earlier, and was fundamental in establishing the Republic. The conspirators felt that with Brutus in tow, recreating the perception that a Brutus would save Rome from tyranny for a second time, the plot would immediately be legitimized. Cassius and other conspirators relentlessly worked on Brutus, trying to shatter his bond with Caesar and bring him to their side. After playing on his sense of duty and family history, Brutus finally joined the plot as a matter of honor.
With the involvement of Brutus, the conspiracy moved forward. All that was left was to determine the time, place and manner of the deed. Caesar was expected to assemble the Senate for a final time before departing for Parthia on March 18. On the Ides of March, or the 15th, the Senate was expected to approve Caesar's kingship outside of Italy. It had been prophesied by the Sybilline books that only a king could defeat Parthia, and it was widely expected for Caesar to receive this additional honor before his departure. It was decided that the assassination would take place in the Curia of Pompey's theatre, where the Senate would meet with Caesar for the last time. Each member of the plot would be required to stab Caesar with a dagger, thereby not only assuring his death, but unifying the group under a banner of freedom for the entire state.
The waiting game, however, must have been a nerve-wracking affair. Bad omens seemed to threaten the success of the attempt. These omens, likely later added manifestations of the writers for dramatic effect, still illustrate the fragile nature of the plot. One little slip up, with so many people involved, could send the whole thing crashing down. Of the many portends, or warnings to Caesar that are said to have occurred, one stands out above the rest. According to Suetonius and Plutarch, a soothsayer by name of Spurinna was said to have delivered the famous warning to Caesar, "Beware the Ides of March."