Having survived a drastic fighting, the soldiers emerge back and refill their lungs with peaceful air, saying nothing about the comrades who passed away. It is via the DEATH IS DEPARTURE metaphor that we interpret the linguistic expression "went under" as referring to the death of the soldiers and their descent into the underworld, which is further reinforced through a combination of the conceptual metaphor DEATH IS DOWN that is based on the VERTICALITY schema, and which is called up via the adverb "under." But why do they keep silent about their fellow soldiers that "went under"? Could it be that they feel guilty for BEING PRESENT HERE when others are gone? Or could it be that the departure of dear ones is bigger than words? The poem did not
