The Liberators' civil war was started by the Second Triumvirate to avenge Julius Caesar's murder. The war was fought between the forces of Mark Antony and Octavian (the Second Triumvirate members) against the forces of Caesar's assassins Marcus Junius Brutus and Gaius Cassius Longinus in 42 BC.
The triumvirs decided to leave Lepidus in Italy, while the two main partners of the triumvirate (Antony and Octavian) moved to Northern Greece with their best troops (a total of 28 legions). In 42 BC Gaius Norbanus Flaccus and Decidius Saxa, were sent by the triumvirs with an eight legions strong advance guard into Macedonia against the murderers of Julius Caesar. In the neighborhood of Philippi, Norbanus and Saxa met the combined advancing troops of Cassius and Brutus. As they were outnumbered, Norbanus and Sax occupied a position near Philipi which prevented the republicans advancing any further. By a ruse Brutus and Cassius managed to make Norbanus to leave this position, but Norbanus discovered the ruse in time to recover the dominating position. When Brutus and Cassius managed to outflank them, Norbanus and Saxa retreated towards Amphipolis. When Mark Anthony and the bulk of the triumvir's troops arrived (minus Octavian who was delayed at Dyrrachium because of his ill-health), they found Amphipolis well guarded and Norbanus was left in command of the town.
The Liberators' army had seventeen legions (eight with Brutus and nine with Cassius, while other two legions were with the fleet). Only two of the legions were at full ranks, but the army was reinforced by a levies from the Eastern allied kingdoms. Appian reports that the army mustered a total of about 80,000 foot-soldiers. Allied cavalry included a total of 17,000 horsemen, including 5000 bowmen mounted in the Eastern fashion. This army included the old Caesarian’s legions present in the East (probably with XXVII, XXXVI, XXXVII, XXXI and XXXIII legions), thus most of his legionnaires were former Caesarean veterans. However, at least the XXXVI legion consisted of old Pompeian veterans, enrolled in Caesar's army after the Battle of Pharsalus. The loyalty of the soldiers who were supposed to fight against Caesar’s heir was a delicate issue for the Liberators. Cassius tried in all ways to reinforce the soldiers' loyalty both with strong speeches ("Let it give no one any concern that he has been one of Caesar's soldiers. We were not his soldiers then, but our country's") and with a gift of 1,500 denari for each legionnaire and 7,500 for each centurion.
The Battle of Philippi consisted of two engagements in the plain West of the ancient city of Philippi. The first occurred on the first week of October; Brutus faced Octavian, while Antony's forces were up against those of Cassius. At first, Brutus pushed back Octavian and entered his legions' camp. But to the south, Antony defeated Cassius, and Cassius, hearing a false report of Brutus' failure, committed suicide. Brutus rallied Cassius's remaining troops and both sides ordered their army to retreat to their camps with their spoils, and the battle was essentially a draw, but for Cassius' suicide.
On the other side, however, the Liberators’ army was left without his best strategic mind. Brutus had less military experience than Cassius and, even worse, he could not obtain the same sort of respect from his allies and his soldiers, although after the battle he offered another gift of 1,000 denarii for each soldier.
In the next three weeks, Antony was able to slowly advance his forces south of Brutus’s army, fortifying a hill close to the former Cassius’ camp, which had been left unguarded by Brutus. To avoid being outflanked Brutus was compelled to extend his line to the south, parallel to the via Egnatia, building several fortified posts. Brutus defensive position was still secure, holding the high ground with a safe line of communication with the sea and he still wanted to keep the original plan of avoiding an open engagement while waiting for his naval superiority to wear out the enemy. Unfortunately, most of his officers and soldiers were tired of the delaying tactics and demanded another attempt at an open battle. Probably both Brutus and his officers feared the risk of having their soldiers deserting to the enemy if they did not keep their ascendancy on the troops. Plutarch also reports that Brutus had not received news of Domitius Calvinus' defeat in the Ionian Sea. Thus, when some of the eastern allies and mercenaries started deserting, Brutus was forced to attack on the afternoon of October 23.
A second encounter, on 23 October, finished off Brutus's forces, and he committed suicide in turn, leaving the triumvirate in control of the Roman Republic. The battle resulted in close combat between two armies of well-trained veterans. Arrows or javelins were largely ignored and the soldiers packed into solid ranks fought face-to-face with their swords, and the slaughter was terrible. In the end, Brutus’ attack was repulsed, and his soldiers routed in confusion, their ranks broken. Octavian's soldiers were able to capture the gates of Brutus’ camp before the routing army could reach this defensive position. Thus, Brutus’ army could not reform making the triumvirs’ victory complete. Brutus was able to retreat into the nearby hills with the equivalent of only 4 legions. Seeing that surrender and capture where inevitable he committed suicide the next day.
The remains of the Liberators’ army were rounded up and roughly 14,000 men were enrolled into the triumvirs army. Old veterans were discharged back to Italy, but some of the veterans remained in the town of Philippi, which became a Roman colony (Colonia Victrix Philippensium).
Antony remained in the East, while Octavian returned to Italy, with the difficult task of finding the land to settle a large number of veterans. Despite the fact the Sextus Pompeius was controlling Sicily and Domitius Ahenobarbus still commanded the republican fleet, the republican resistance had been definitely crushed at Philippi.