They are confident of victory. They are aware of the opposing forces’ weaknesses. They believe Brutus and Cassius have made a mistake in coming to Philippi.
Battle of Philippi, (3 and 23 October 42 BCE) was the climactic battle in the war that followed the assassination of Julius Caesar in 44 BCE.
Philippi saw the final destruction of those who favored the old Republican constitution of Rome. The battle was a brutal killing match with much confusion and little generalship on either side.
Historically, there were two battles at Philippi but Shakespeare merges them into one in his play Julius Caesar, in five continuous scenes, without a break (background of the story in Acts 5).
On the battlefield at Philippi, Antony and Octavius agree to their battle plans. They meet with Brutus and Cassius before entering battle, but only exchange insults. Battle is imminent. All four men return to their armies to prepare for war.
In the middle of the battle Brutus sees a chance to destroy Octavius' army and rushes away to attack it. He leaves Cassius behind. Cassius, less militarily adept, quickly begins losing to Antony's forces. Even worse, Pindarus misleads him, telling him Titinius has been taken by the enemy near Cassius' tents. Upon hearing this news, Cassius orders Pindarus to kill him. After completing the task, Pindarus flees. Brutus arrives, finds his friend dead and remarks, "O Julius Caesar, thou art mighty yet."
See a summary and analysis of Act 5 (via GradeSaver.com).