Battle of Philippi - Wikipedia

On October 23, 42 BC, during the Roman Republican civil wars, in the second engagement of the Battle of Philippi, the Army of Brutus was decisively defeated by Mark Antony and Octavian. Brutus commit suicide. From the article:

"The Battle of Philippi was the final battle in the Wars of the Second Triumvirate between the forces of Mark Antony and Octavian (of the Second Triumvirate) and the leaders of Julius Caesar's assassination, Marcus Junius Brutus and Gaius Cassius Longinus in 42 BC, at Philippi in Macedonia. The Second Triumvirate declared this civil war ostensibly to avenge Julius Caesar's assassination in 44 BC, but the underlying cause was a long-brewing conflict between the so-called Optimates and the so-called Populares.

The battle, involving up to 200,000 men in one of the largest of the Roman civil wars, consisted of two engagements in the plain west of the ancient city of Philippi. The first occurred in the first week of October; Brutus faced Octavian, while Antony's forces fought those of Cassius. The Roman armies fought poorly, with low discipline, non-existent tactical coordination and amateurish lack of command experience evident in abundance and neither side able to exploit opportunities as they developed.[3][4] At first, Brutus pushed back Octavian and entered his legions' camp. But to the south, Cassius was defeated by Antony, and committed suicide after hearing a false report that Brutus had also failed. Brutus rallied Cassius' 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. A second encounter, on 23 October, finished off Brutus's forces after a hard-fought battle. He committed suicide in turn, leaving the triumvirate in control of the Roman Republic."