To me, the theme of this poem is how war is totally opposed to how human life is supposed to be. Equally important, though, war is not enough to keep people from being people.
For much of the poem, we see the first theme. The beauty of the snow is marred by all the terrible things of war, including the dead bodies. To me, the snow can be seen as a symbol for what human life should be -- beautiful.
But then at the end, we see that (for one moment at least) the guard's humanity overcomes the war and he can be a person enjoying things that people should enjoy.
The poem 'First Snow in Alsace' by Richard Wilbur presents, as one of its themes, how two interpretations of reality can juxtapose into a comparison that is thought-provoking and almost metaphysical. For example, yes the scene is beautiful albeit in a subjective way. Yes, there is a covering of snow and that is white and silvery and silent and beautiful. Those things are hard facts and are real. Yet it also real that there is dirt, and pain and weapons and death in the same environment. The contrast is hard to take in. It is hard to comprehend, both for the poet and for us. So he writes about it, trying to thrash out how the the two images can co-exist. In some ways, the work has Nature Poetry elements - we cause the mayhem, but powerful Nature goes on regardless.
Alsace is the furthermost region of eastern France that borders Germany. The region was annexed by Germany in 1940 during World War II.
The theme of the poem deals with the snow as a brief, beautiful interlude during World War II. The snow is like a huge blanket dressing that covers the wounded landscape. Gone are the borders, the carnage, the charred landscape: all that remains is "absolute snow."
The poem ends with the night guard who remembers "ten first-snows back" to when he was age 10 or so. He even takes pride in the memory and being the first to witness this snow.
So says Enotes editors:
The gap between human pettiness and natural beauty, the youthful innocence of the combatants, the preciousness of a moment’s joy in a time of fear and despair—all these themes emerge from the precise but emotionally loaded description of “First Snow in Alsace.”