Led by Lucius, the Roman Army faces the British Army. The two sides march across the stage and exit, as a way to mimic the movement of troops in battle. Dressed up as a poor British soldier, Posthumus enters the stage again, fighting with Iachimo. Posthumus wins the fight, disarms Iachimo, and leaves.
The battle begins with pure stage business and does not include any dialogue. This part of the scene is not unlike the dumb show—an Elizabethan device where actors would mime dramatic events—and conveys the violence of the battle in a time before advanced special effects. That the disguised Posthumus defeats Iachimo sets the tone for an ultimate reckoning between the two, and hints that Iachimo will have to reconcile his treachery.
Now that he is back in Britain, Iachimo feels overcome with sadness and guilt for lying about the British princess. He swears that even the harsh British climate is trying to take revenge on him for betraying Imogen. If the air hadn’t made him weak, he would have been able to defeat the British peasant he just fought with (who, as it turns out, was Posthumus in disguise). Iachimo concludes that noble titles and knighthood must not have any real value, because a peasant with no title fought so well against him. What’s more, if a peasant can fight that well, then the British nobility must be god-like in their fighting abilities. On that note, lachimo exits.
Iachimo vocalizes his guilt—a guilt so overwhelming that he even projects it onto the British environment. Much like Posthumus returning to Britain with a repentant heart, Iachimo—formerly a swaggering, braggadocious character—feels the weight of his immoral actions. These two characters’ remorse—in addition to Cymbeline’s softening at the end of Act 4—gestures towards an ultimate reconciliation befitting the play’s genre as a tragicomedy. Iachimo also echoes Posthumus’ comments on nobility—that appearances and innate nobility don’t always match.
The battle rages on. The British soldiers retreat, and the Romans capture Cymbeline. All seems lost when suddenly Belarius, Guiderius, and Arviragus enter to rescue the British King. Belarius encourages his sons to fight—they have the advantage of knowing the terrain, and they can hide themselves in a lane carved out of the landscape—a sort of trench—so that the Romans can’t ambush them. The only thing that can stop the brothers from successfully freeing Cymbeline is their own fear. Posthumus re-enters and joins Belarius, Guiderius, and Arviragus—they rescue Cymbeline, and leave the battle.
Though he was initially reluctant to even enter into the battle, Belarius feels emboldened. He is the one to encourage his adopted sons to stand their ground, when they had previously been the ones to encourage him to fight at all. Their display of courage is the last-ditch effort needed to defeat the Romans—and the Britons succeed. The outcome of the battle is another reversal of audience expectations, like Posthumus and Iachimo’s repentance. The defeat is a major blow to the warlike and organized Romans, who are taken down by a ragtag bunch of British soldiers, and whose Empire’s future now hangs in the balance.
Lucius, Iachimo, and Imogen (still disguised as Fidele) enter. Lucius urges Fidele to run away from the fighting to save himself, because the battlefield has descended into chaos. Soldiers are turning against comrades on their own side, and the fighters are stumbling into disorder as if they were blindfolded. Iachimo insists that the new troops are causing the trouble. Lucius remarks that the day took a strange turn: even though it seemed like the Romans would win the fight, the Britons have proved victorious. Lucius tells Iachimo that they should help their fellow soldiers or else retreat.
Lucius confirms that the battle has taken an unexpected turn. Troops who had been solidly trained have turned on their own comrades-in-arms, and are fighting poorly. With such extensive preparations and the Soothsayer’s prediction of victory, Lucius is right to be stunned by the Romans’ loss—an unexpected twist of fate that could see a part of the Empire successfully winning its independence.