Post WWI was more cynical and realistic showing of war where Post WWII contained messages of hope or futuristic warnings of dictatorships.
Books like "All Quiet on the Western Front" which was written after WWI, showed the real devastation and conditions of the trenches. They warned of the nature of war itself. Whereas, books following WWII, like 1984 and Fahrenheit 451, warned of extreme governments in which civil liberties are threatened.