For the Senate and People of Rome-How Pompey, Cato, and Cicero Saved The Republic

Pompey's Triumph In The Civil War
The god of war hates those who hesitate.
-Marcus Tullius Cicero

Bust of Pompey
In the summer of 48 BC, the fate of the res publica hung in the balance. In an enormous gamble, Caesar had crossed the Rubicon, and invaded Italy, after being ordered by the senate to surrender his command and return home. Pompey, who had pledged to defend Italy, fled along with many prominent senators, to the heel of the peninsula. Caesar desperately pursued him, knowing if Pompey escaped Italy with his army, he would be in a terrible position. Showing his logistical and military brilliance, Pompey managed to just make it out of Italy with his army, before Caesar could snap the jaws shut in Brundisium. Now in Epirus, Pompeius Magnus had all the riches and manpower of the east at his disposal. The east was where he had made his name. Now it was where he would defend it, along with the republic.
Caesar had moved rapidly down through Italy. Showing clemency to the Italians and to the soldiers facing off against him, he would offer them the opportunity to join him, or just let them go their own way. For the commanders, he would also dismiss them and let them go free. This was clearly a calculated move by Caesar, in an attempt to win over the populace in Rome and Italy, and to prove that he was not another Sulla. Deprived of his chance to end the civil war quickly upon Pompey's escape, he turned his attention to solidifying his control of the western Mediterranean.
In an astonishingly fast 27 day march, Caesar defeated Domitius Ahenobarbus (whom he had released after the latter's surrender at Corfinum) at Massilia, and then crushed the politically leaderless Pompeian army at Illerdia in Hispania. His force of six legions and 3,000 cavalry, along with his 900 man bodyguard proceeded with their pacification of Hispania, suffering 70 men killed, while the Pompeian forces suffered 800 casualties, 200 dead and 600 wounded.
Returning to Rome in December of 49 BC, he was appointed dictator by the senators who had remained in Rome, with Mark Antony as his Master of Horse. He only kept this position for 11 days, enough to win the consulship along with Servilius Vatia Isauricus. He then renewed his pursuit of Pompey, who himself was gathering and training men, and feeling extremely confident in his chances.
Caesar's Last Stand
The gods favour the bold.
-Ovid

Caesar leading his men at Dyrrachium
Bibulus was charged with the task of preventing the Caesarion forces from reaching Greece, and Pompey had every reason to believe Caesar could not successfully cross. Lacking a fleet, Caesar skimmed for ships, and attempted a winter crossing of 15,000 soldiers from Brundisium to Palaesa in Epirus. Bibulus was caught off guard, and the crossing was successful, with Caesar establishing a beachhead.
Bibulus was now prepared though, and prevented Caesar's ships from returning to Brundisium to retrieve the other half of his army. Many of Caesar's ships were burnt, and he was left in a precarious state. Bibulus fell ill and died shortly thereafter.
Now Caesar was in the worst possible state an army could possibly be in. Having been cut off from supply from Italy, he could not count on the locals to supply him, as Pompey's eastern glories had won over their hearts, making the Greeks staunchly pro-Pompey. He was in such a desperate state, that he tried multiple times to make peace with his adversary, but was refused at every attempt. Coming to the realization he would have to fight his way out of this precarious position, he attempted another winter blockade run, but rough seas and high winds forced the ships back.
Mark Antony rallied his men in Italy, and successfully broke through the blockade, landing much needed troops on the Epeirote coast. Now it was a race between Pompey and Caesar to reach Mark Antony first. Pompey got to Antony first, with Caesar hot on his heels. Pompey broke off and moved his forces to Dyrrachium, wisely avoided being caught between Caesar and Antony.
Pompey now commanded a near perfect position. With his back to the sea and naval superiority, he could supply his men with impunity. The immediate area was surrounded by hills, making direct assault virtually impossible. Caesar pulled a play from the siege of Alesia, and ordered fortifications to be built to pin the Pompeian forces against the sea. Pompey's response was a system of walls and fortifications of his own to prevent any further Caesarion advancement. Constant indecisive skirmishes took place in no-man's land in between the two fortified positions. While Caesar had control of the surrounding farmland, Pompey had made sure it was picked clean of food, and through the sea, could be supplied constantly. As the harvest approached however, Pompey knew Caesar's men would have the necessary food to continue the blockade, and Pompey was not sure if he could maintain a sufficient amount of fresh water needed.
Then, in what would be known as the defining moment in the civil war, two gallic auxillaries, having escaped punishment for stealing pay from Caesar's legionaries, arrived in Pompey's camp. They informed Pompey that a section of Caesar's wall was still uncompleted and was the only plausible option for attack.
Pompey mounted an attack of six legions against Caesar's line where it joined the sea and where Caesar's IX legion was stationed. Vastly outnumbering Caesar's men, Pompey's forces broke through, forcing Caesar's men to pull back. Caesar swiftly counterattacked with 12 cohorts and drove the Pompeian forces back. Pompey's forces were too large however, and what was initially a successful counterattack, became outflanked on the right, and was beginning to buckle. His wings began to collapse, and Caesar's army began to panick and rout. Caesar realized the potentially disasterous situation his army was in and attempted to coordinate an organized withdrawal.
Pompey ordered his men to push on [1] feeling this was his chance to crush Caesar and end the civil war once and for all. The organized retreat turned into a full on rout, and Caesar's forces were crushed. What remained of the Caesarion forces limped back and set up a last ditch defense, where Caesar himself was slain along with many of his men. In an act of clemency, Pompey offered the Roman legions to rejoin him and the republic instead of face annihilation, which was readily accepted. In one master stroke, Pompey The Great had crushed Caesar and preserved the republic, cementing his legacy as the saviour of the republic in Roman history.
[1] In real life, Pompey feared a trap and thought Caesar was finished, and halted his men. This allowed Caesar's men to regroup, eventually winning at Pharsalus.