There is a sense in which the play deals with the theme of heroism. There is heroism based on idealism. This kind is manifested by Sergius, a proud upper class figure, who is very conscious of his aristocratic background, coming, as he does, from a family with the highest rank in the army.
Sergius's claim to heroism lies in his leading the charge against the Serbian army during the war between the two nations. His exploits are recounted as the Serbian army is routed and he returns home to earn a promotion on that account. That exploit also earns him the adoration of his fiancée, Raffia, as well as her mother, Catherine. However, it takes Captain Bluntschli on the Serbian side to give an account of the true events on the battlefield to downgrade Sergius's achievement. On the battlefield he showed more bravado than bravery and lost more men than should have been the case, especially when the Serbians had less ammunition.
Sergius pays dearly for what could have made him a real hero. He loses both rank and fiancée as a consequence of his foolhardy handling of the war.
But there is the other kind of the hero represented Bluntschli. His opposition to war makes him carry chocolate in the cartridge of his gun, instead of real bullets. And he saves himself when all appears lost by seeking refuge in Raina's bedroom. What appears to be an unheroic act turns out to be the more realistic option. It is his forthrightness and practical mindedness that earn him Raina's love and their eventual marriage.
Bluntschli's pragmatism and military skill also make him heroic. While Catherine is at her wits' end as to how to get rid of him, Sergius and Petkoff are racking their brains as to how to dispatch the three calvary regiments to Philippopolis. Bluntschli comes to the rescue, bringing relief to the two whose already high regard for Bluntschli rises even higher.
Yet, there is the other act of heroism which is illustrated by Louka. Discontent with her lower class status, she vows to improve her status by fair or foul means. She ensures she breaks the relationship between Sergius and Raina by eavesdropping on Raina's conversation with Bluntschli and reporting same to Sergius. Eventually, she turns Sergius's attention towards her and they both break class barriers by marrying. That is a heroic achievement in a class conscious society.