January–March 49 BC · Italy Theater

Caesar's crossing of the Rubicon with a single legion on January 10, 49 BC was met with near-total collapse of Pompeian resistance in Italy. Pompey and the Senate had expected Caesar to negotiate — he had been making war for eight years in Gaul and his troops were veterans while many Pompeian garrison soldiers were raw recruits. Town after town in northern Italy opened its gates to Caesar without a fight. Marcus Crassus, commanding a garrison force at Corfinium, was abandoned by his own soldiers before Caesar arrived and had to surrender the entire force — whom Caesar pardoned and incorporated into his own army. Pompey retreated southward to Brundisium (modern Brindisi), where Caesar caught him and attempted to blockade the harbor with an enormous mole. But Pompey's engineers held off long enough for his army and most of the Senate to embark for Greece. Within 60 days of the Rubicon crossing, Caesar controlled all of Italy without a major battle.

June–August 49 BC · Hispania Citerior (Spain) Theater

While Pompey organized the eastern legions in Greece, Caesar turned to eliminate the seven veteran Pompeian legions in Spain under Afranius and Petreius — 'an army without a general,' he called them, intending to deal with Pompey afterward as 'a general without an army.' The campaign around Ilerda (modern Lleida) on the Ebro was Caesar's most patient operation: he maneuvered to cut off water and supply rather than fighting pitched battles, outmaneuvered the Pompeian position through a series of forced marches and river crossings, and eventually trapped the entire Pompeian force without food or water. When Afranius and Petreius attempted to march to the Ebro, Caesar surrounded them on a waterless plateau. After five days the Pompeian soldiers began deserting in large numbers. The commanders surrendered.

June–August 48 BC · Epirus (modern Albania) Theater

Caesar crossed the Adriatic in January 48 BC with only half his army, deliberately risking surprise. He established a position near Pompey's camp at Dyrrachium and then, in a remarkable reversal of the Alesia strategy, attempted to surround Pompey's far larger army with circumvallation — though this time on open coastal ground rather than hills. Both armies dug increasingly elaborate earthworks for months. Pompey had naval superiority and could resupply by sea; Caesar's men were eating bark and roots. In late July, Pompey found a gap in Caesar's lines — betrayed by two Gallic officers who deserted — and launched a coordinated attack that collapsed Caesar's left wing. For the first and only time in the civil wars, Caesar ordered a retreat from a battle. He lost approximately 1,000 men killed and 32 centurions. Pompey failed to press the pursuit, and Caesar reportedly said that the war might have ended that day if Pompey knew how to win.

August 9, 48 BC · Thessaly (central Greece) Theater

The battle that destroyed the Roman Republic was fought on the plain of Pharsalus in Thessaly on August 9, 48 BC. Pompey had a significant numerical advantage — perhaps 45,000 infantry to Caesar's 22,000 and 7,000 cavalry to Caesar's 1,000. His plan was simple: use his overwhelming cavalry on the left wing to sweep away Caesar's horsemen, then envelop the Caesarian infantry from the rear. Caesar anticipated exactly this and placed six secretly withdrawn cohorts behind his cavalry with orders to advance obliquely when the Pompeian horse charged. The plan worked precisely: Pompey's cavalry, unused to infantry counter-attacking from unexpected angles, broke and fled. The six cohorts then wheeled into the Pompeian infantry's unprotected flank. Caesar's veteran infantry, advancing simultaneously from the front, completed the encirclement. The Pompeian army collapsed entirely; Pompey fled the battlefield without giving any orders, according to Caesar's own account. Caesar reported 15,000 Pompeians killed and 24,000 prisoners; his own losses were 200 soldiers and 30 centurions.

October 48 – March 47 BC · Alexandria, Egypt Theater

Caesar followed Pompey to Egypt and arrived to find him already murdered. He installed himself in the royal palace at Alexandria with only 3,200 men — far too few to safely occupy a hostile city of a million people. The Egyptian court, ruled effectively by the eunuch Pothinus and the general Achillas, had murdered Pompey to please Rome but resented Caesar's occupation and his support for the exiled queen Cleopatra. She famously had herself smuggled into the palace in a rolled carpet — or a linen sack according to some sources — to present herself to Caesar personally, beginning a political and romantic alliance that would last until Caesar's death. When Achillas's army of 20,000 surrounded the palace quarter, Caesar was trapped for months. He burned Ptolemy's fleet in the harbor — the fire spreading to some buildings on the shore, though not, as later legend claimed, to the famous Library. Reinforcements finally arrived; Ptolemy XIII drowned in the Nile trying to escape. Caesar spent two more months in Egypt before departing, leaving Cleopatra pregnant with a son, Caesarion.

May 2, 47 BC · Pontus (northern Turkey) Theater

Returning from Egypt via Syria, Caesar was confronted by an unexpected crisis: Pharnaces II, son of Mithridates the Great, had taken advantage of the Roman civil war to reconquer his father's old kingdom of Pontus in northern Anatolia, defeating a Roman army at Nicopolis and capturing Roman allies. Caesar marched to Pontus with three legions, established a camp near Zela (modern Zile, Turkey) — the site of a previous Roman victory over Mithridates — and prepared for a siege. Pharnaces attacked immediately and uphill, apparently believing the terrain would negate Caesar's tactical superiority. The battle was furious and briefly dangerous — Pharnaces's scythed chariots caused some confusion — but Caesar's veterans broke the Pontic army in about four hours. Pharnaces fled, abandoned his kingdom, and was later murdered by a former subordinate. Caesar reported the victory to Rome in a letter that became one of the most famous dispatches in history.

February 6, 46 BC · Africa (Tunisia) Theater

After Pharsalus the surviving Pompeian leadership regrouped in the Roman province of Africa, where they had the support of King Juba I of Numidia and a significant army including war elephants — a weapon Caesar's veterans had not faced since Thapsus's namesake battle in the First Punic War. Caesar crossed to Africa with only 3,000 men to start with, suffered early reverses, and had to be resupplied before he could fight. Near the town of Thapsus, Caesar maneuvered Scipio's army into an indefensible position. When the battle began, Caesar's veterans acted without orders — the trumpets on his right wing sounded and the soldiers simply charged before Caesar could stop them. Caesar later said he had not given the order but the outcome was never in doubt: the Numidian elephants panicked under fire and fled back through their own lines, and the Pompeian infantry was broken and pursued without mercy. Scipio and Afranius were killed trying to escape; Cato the Younger, commanding at nearby Utica, refused either to fight or to accept Caesar's now-predictable pardon, and stabbed himself.

March 17, 45 BC · Hispania Baetica (Spain) Theater

Pompey's sons, Gnaeus and Sextus, had rebuilt a resistance force in Spain using local Spanish allies and legions loyal to their father's memory. Caesar arrived in Spain in November 45 BC and engaged the younger Pompeians at Munda near modern Osuna. The battle was the hardest-fought of Caesar's career — fought uphill against a prepared position, where the veteran soldiers on both sides pressed each other so close that artillery could not be used and there was no room to throw pila. Twice during the fighting Caesar told his staff that he had often fought for victory but today he had fought for his life. The crisis broke when Labienus — Caesar's old legate who had defected after Pharsalus — moved troops laterally to counter a cavalry threat, and the Pompeian infantry thought the movement was a retreat. Their formation broke. Labienus was killed in the fighting. Gnaeus Pompeius fled and was caught and killed weeks later; Sextus Pompey escaped and would trouble Rome for a decade.

April 14–21, 43 BC · Cisalpine Gaul (northern Italy) Theater

After Caesar's assassination on the Ides of March 44 BC, Mark Antony seized the initiative — acquiring Caesar's papers, his money, and the loyalty of his veterans. His political position became untenable when Octavian, Caesar's 18-year-old great-nephew and heir, arrived in Rome and began recruiting Caesar's veterans by paying them from his own inheritance. The Senate, manipulated by Cicero who hoped to use Octavian against Antony, declared Antony a public enemy and sent both consuls (Hirtius and Pansa) north with Octavian to relieve Decimus Brutus, whom Antony was besieging at Mutina. Two battles were fought on successive days: in the first Antony was defeated by Pansa's army, though Pansa himself was mortally wounded; in the second Antony defeated Hirtius but Hirtius was killed. Both consuls died within days — conveniently for Octavian, who was left in sole command of the Republican forces he had just been fighting alongside.

October–November 42 BC · Macedonia (northern Greece, near modern Kavala) Theater

The final reckoning for Caesar's assassination came at Philippi in Macedonia, where the Liberatores — Brutus and Cassius with 19 legions — faced the Second Triumvirate's 22 legions under Antony and Octavian. Two battles were fought three weeks apart. In the first, on October 3, Antony broke Cassius's wing and overran his camp; simultaneously, Brutus broke Octavian's wing and overran his camp. The symmetrical result left both sides roughly intact, but Cassius — unable to see what was happening on Brutus's side through the dust — believed the entire battle lost and committed suicide. In the second battle, November 23, Brutus was pressured into attacking prematurely when desertions eroded his position. His army was broken; Brutus escaped with a small force but killed himself that night. His last reported words were a line from Euripides: 'O wretched virtue, thou wert but a name, and yet I worshipped thee as real indeed.' Octavian reportedly had Brutus's head cut off to lay at the feet of Caesar's statue.

September 3, 36 BC · Sicily (Strait of Messina) Theater

Sextus Pompey had spent years building a naval empire in the western Mediterranean, using Sicily as a base to blockade Italian grain supplies and shelter refugees from the civil wars including escaped slaves and political enemies of the Triumvirate. His fleet was formidable — he had the best sailors in the Roman world and the propaganda advantage of invoking Neptune as his patron. Octavian had been embarrassed twice in naval operations against Sextus. The answer was Marcus Agrippa, the greatest naval commander Rome had produced, who spent a year building and training a new fleet at the artificial harbor of Portus Julius near Naples. At Naulochus off the northern Sicilian coast, Agrippa deployed a new weapon: the harpax — a grappling catapult that fired an iron hook on a chain to seize enemy ships and winch them alongside for boarding. The combination of the harpax, superior training, and the demoralization of Sextus's sailors after months of naval setbacks was decisive. 28 of Sextus's 300 ships escaped; the rest were sunk or captured.

September 2, 31 BC · Epirus (western Greece, Gulf of Ambracia) Theater

The final battle between Antony and Octavian was decided before it was fought. Antony had wintered his army of 100,000 infantry and 500 ships at Actium on the Gulf of Ambracia, but disease and desertion had gutted his forces throughout the summer of 31 BC as Agrippa systematically seized the ports and supply routes of the Adriatic and Mediterranean. By September Antony's army was too weakened to retreat overland and his naval crews too diminished to fight an offensive naval engagement. The battle was essentially a breakout attempt: Antony loaded his treasure ships and tried to run through Agrippa's blockade with the best of his remaining fleet. Cleopatra's squadron of 60 ships successfully broke through when a gap opened — either because she followed a prearranged plan or because she fled, depending on which source you believe. Antony then abandoned his flagship and chased after her. His remaining fleet, leaderless, surrendered within hours. His army on land surrendered a week later.