The poem is called "The Soldier," so naturally it's about… war. Unlike many other famous World War I-era poems, however, Brooke paints a more optimistic picture. The soldier's possible death is mentioned, yes, but so is a blissful life after death. Moreover, the poem celebrates the fact that the soldier's death will give England another "corner" of land. So, for the speaker, all this warfare business seems like a big win! Of course, he hasn't actually been to war just yet….
Yeah, uh… no thanks. War is unimaginably horrific, and this is why the poem refuses to discuss it directly.
War is so influential in this poem that the only imaginable peace is the peace of heaven—the peace we achieve after death. Bummer.