In 42 BC, Octavian and Antony combined their forces, 28 legions in total, and sailed across the Adriatic and into Greece. The 'Liberators' Brutus and Cassius had 19 of their own legions, which were heavily supplemented by auxilia provided by eastern client kingdoms.
Brutus and Cassius had been plundering and taking control of the east for nearly two full years since the murder of Caesar. Despite having an army made up largely of Caesar's former troops, they used this plunder and distributed it among the men to secure their loyalty.
As Octavian's and Antony's armies arrived and assembled near Dyrrhachium, the site of Caesar's near defeat to Pompey 6 years earlier, Octavian battled with his own poor health. Often described as a sickly youth, he was apparently stricken with a terrible illness just as the fate of the Roman world was about to be decided. Antony, however, likely saw a grand opportunity to win a great victory for his own cause without being forced to share any credit with his young fellow triumvir and rival. As Antony marched his army east towards the Macedonian - Thracian and border and confrontation with the enemy, Octavian had no choice but to follow, despite his illness, or risk being left out of the battle to revenge his adoptive father.
Initially both sides jockeyed for position, and the 'Liberators' hoped to win a battle of attrition by delaying Antony's advance. Antony however, often considered second only to Caesar in military ability during this era, would have none of it and forced the enemy into battle. On October 3, the two armies drew up near the Macedonian town of Philippi. Cassius commanded the left wing of the Republican forces directly across from Antony while Brutus confronted Octavian's army with the right wing. Octavian, however, despite his presence in the area was still terribly ill.
He was forced to stay behind the lines in his tent, while his officers conducted the battle on his behalf. As the battle opened, Antony had a clear advantage over Cassius, and overran the Republican left. Brutus, though, had nearly equal success against Octavian and pushed his lines back. Octavian was forced to flee his camp, taking refuge in a nearby marsh.
Cassius' defeat was significant and yet the entire affair could've been stabilized by Brutus's success. Cassius though was certainly unaware of his ally's good fortune and decided to take his own life, rather than submit to Antony. Despite his own loss, Cassius was the stronger military mind of the Republican side and his own death began to sound the end of their ability to resist. Brutus managed to regroup and take command of Cassius' remaining army, but the writing was on the wall. Antony assuredly reveled in his own victory while Octavian was forced to retreat, but Brutus held his ground and delayed Antony's triumph. On October 23, perhaps losing the confidence of his men, or willing to risk a final last ditch effort at victory, Brutus launched an attack.
At the Battle of 2nd Philippi, Octavian was seemingly recovered from his illness and commanded his own army. He and his men were certainly embarrassed by their defeat just 3 weeks earlier and were prepared to give a better account. This time they proved themselves up to the challenge, and the triumvir's army overran Brutus. Octavian's forces captured Brutus' camp and they were atoned for their previous defeat.
The battle spelled the end of the Republican cause, and Brutus committed suicide on the following day. A great number of those involved in the plot against Caesar also lost their lives at Philippi and Octavian was brutal in exacting vengeance. Though some escaped to join with Sextus Pompey in his Sicilian stronghold, the battles of Philippi essentially assured the end of Republican government and paved the way for a final conflict between the victorious triumvirs.
After the battles, Octavian marched his army back to Italy, where he was now faced with the unenviable task of finding a retirement settlement for his veterans. Antony continued east where he began to secure loyalty of client kings and provincial governors alike. He imposed serious penalties on Asia Minor in particular and essentially plundered those provinces for disloyalty, despite already having been looted by Brutus and Cassius.
In light of the altered state of the Roman world, the triumvirs realigned their positions. Antony received the entire east as his new territory, yet retained Transalpine Gaul. Octavian now moved into the second position among the three received Spain, Italy, Cisalpine Gaul and the Mediterranean islands. Lepidus, clearly relegated to third on the list, was moved to Africa, where he would essentially linger as a bit player in the remaining days of the Republic.