I guess it depends on the definition of song, and also on the correspondence under consideration.
My first instinct is to say “Quatuor pour la fin du temps, duh," and I still think that’s probably the best answer, in terms of high-art response to a terrible, disruptive event in mid-twentieth-century European history that uses the techniques of modernism to express the anguish of personal and communal suffering. But although my definition of "song" is elastic enough to include any piece of music complete in itself, the Quartet is not scored for voice, and cannot therefore be considered a song according to traditional Western musical taxonomies.
So if we’re limiting ourselves to pop-music equivalents, something like "Desolation Row" might come to mind: the fragmented imagery, the impish sense of humor even among the horrors of life, and the generous communitarian spirit could link the two.
But I am not generally a fan of the school of thought which believes, or pretends to believe, that modernism did not enter popular music until the rock era. (How anyone who has heard Cole Porter, Kurt Weill, Noël Coward, or Porgy and Bess can imagine this to be true is beyond me; the only possible excuse I can imagine is overfamiliarity.) So my last nomination, and best (if Messiaen, or Penderecki, or Varèse don’t count), is "Strange Fruit" as performed by Billie Holiday, particularly the original 1939 acetate. As a response by a very great artist to a particular horror of modern life, it is at least as great as Guernica; as a technical construction it may not quite measure up to it (if we’re talking things like lyric and melody); but as a conscious expression of skilled emotional artistry it surpasses it.