The first act of Shaw's Arms and the Man sets the plot of the play in motion, and introduces us to the main characters and themes. In the first act, we encounter the two major areas of conflict that are the central themes of the play:
War: The main conflict over war is one of a romantic versus a realistic approach, the former expressing patriotic sentiment and a literary and artistic tradition and the latter exemplifying the pragmatic and mercenary approach to war as a business having as its goal not nobility but surviving and winning. Sergius, the Bulgarian aristocrat and patriot, exemplifies the former and Bluntschli, the Swiss mercenary, the other.
Love: Just as there is a tension between the two views of war, so too is there a tension between romance as represented by Sergius, which is based on literary tradition and romantic illusion, and real love which is grounded in honesty, intellectual compatibility, and mutual practical and emotional support, which Raina eventually discovers with Bluntschli.
As Raina begins to be influenced by Bluntschli's account of the battle, we get the basic tension that makes the plot interesting. As an audience, we are fascinated by the education of Raina. The first act builds suspense as we wonder whether her new understanding will make her see Sergius in a new light and perhaps reconsider their relationship; the ending of the play of course resolves the tension, and even has Sergius show his own discomfort with the heroic role into which he feels he has been compelled.
The first act is the opening scene of the play. Any opening scene is supposed to introduce some of the main characters as well as the main theme(s) of the play. Here in Shaw's Arms and the Man, we are introduced to the Petkoff household which is the place of dramatic action, and also to the romantically disposed young Petkoff daughter, Raina, who is going to be one of the majormost dramatis personae. The Serbian artillery man, Bluntschli, who secretly enters Raina's bed-chamber through the balcony, is the Shavian spokesman in the play, the professional soldier as opposed to the "hero of Slivnitza" Sergius Saranoff. We come across Raina's mother, Catherine, who informs her daughter of the sensational Bulgarian cavalry charge as led by Raina's betrothed love, Sergius, her description of the glorious victory of Sergius being soon undercut and terribly mocked at by the fugitive Serbian mercenary. The scene thus introduces both the theme of war and soldiering and the theme of romantic-sentimental love as divorced from real life. The scene ends with a newly-aroused sympathy on the part of Raina for the fugitive having fallen asleep in Raina's bed.