War art

Art has taken war as subject matter for as long as there has been art (exactly how long that is remains up for debate). Think of the Hellenistic Dying Gaul or Trajan’s Column. War ceased to be central subject matter at about the time artists decided that they didn’t need any subject matter. So there's a chicken-or-the-egg debate for you.
During the 1800s, artists painted dramatic scenes of combat and uprising, spoils of war, victory, defeat, etc. These paintings were sometimes critical, other times supportive of the crisis at hand. Frequently of monumental scale, these works universally convey the importance of the subject matter, the heroism or villainy of the individuals depicted. War was serious, important business, and art knew it.
I do not know if there are such paintings of the first World War. Maybe there are, but if so, I’ve never seen or heard of them. Many artists who personally experienced the horrors of the first world war returned to their easel and brushes with disgust, anger, disillusionment, and trauma. Their images ache with the pain of innocence lost. They didn’t paint the importance of war, but the personal (or impersonal) consequences of it.
However painful the reality of the first World War was, it could not compare with the second. All of Hiroshima and Nagasaki disappear in a blinding flash, millions of European Jews are systematically exterminated, suicide pilots on death missions and so on. How, pray tell, are you going to represent that in the prim art of painting? Picasso painted Guernica, but I don’t know that anyone else even tried. Faced with all that, I think I’d choose non-representationalism too. It is a simpler solution.