The speaker feels worn down by their own sense of purpose, which is described as "limp" and small. It no longer drives them; it tires them. From there the poem turns to the image of the canary in the mine, the bird that warns of danger. The speaker sees themselves as that warning voice, but the mine is "a prison of my own," so the trap seems partly self-made.
Meanwhile, the world outside is violent. The speaker hears "both sea and soldiers" using "their great guns," placing the natural and the human in one threatening noise. Against that din, the speaker's own voice seems very small.
The ending is quiet and a little bleak: the speaker has "nothing almost" to sing about. The word order is odd, and it leaves a faint flicker rather than total silence. The title, "Poetry makes nothing happen," fits that doubt about whether a song can change anything. The mood is tired, uneasy and restrained, with a thin thread of persistence.