The poem is only a few lines long, and it leaves a lot unsaid. Its first sentence is strange on purpose: "joy" seems to act like a person, giving "other instructions" and then handing the speaker the task of building a ship. The way it's phrased, joy feels less like a freely chosen feeling and more like an authority issuing commands. A ship also suggests voyage, departure, or perhaps military use, though the poem never says which.
The second stanza shifts into a quieter, more bitter realization. The speaker "soon" understands that the war "had a lying use." The poem doesn't spell out the lie, but the title, "For god and country," points to the big, noble causes used to justify fighting. The speaker seems to see through that language. The mood is cool and disillusioned, with a hint of dread: the speaker has been put to work, and only afterward grasps how they were deceived.