I would say that this poem is trying to tell us that war is sad but it is also in some way honorable or at least necessary.
We can clearly see that war is tragic in the first two stanzas. The speaker talks about how he and the rest of the dead were vibrantly alive just a few days ago. They could enjoy nature, they could love and be loved, but now they are dead because of the war.
But that is not all there is to war. The last stanza shows that the war is important and that they are not upset to have died. Instead, they seem to feel proud of their actions and they expect the living to continue the fight.