This is such a broad topic that many things can be stated. In addition, there will also be many differences of opinion. Here are some points that might help you think about this request.
First, one of the most interesting points is that most of the war novels after World War I were not written by Americans, but Europeans. For instance, one of the most famous novels was written by Henri Barbusse on the theme of anti-war (Le Feu). Other notable authors were: Ernst Junger and Erich Remargue.
Second, when American wrote, they focused on the outcome of war. For example, Virginia Wolf wrote about the psychological trauma that war produces (Mrs. Dalloway). Ernest Hemingway's Farewell to Arms also deserves to be mentioned here as well.
With the completion of World War II, there was a new spate of war novels. This should not be surprising, because there was much support for World War II. the themes though, if we can speak generally, gave an intensely personal perspective on war. We can say that there was a individualism that emerged in these works. We could also say that other themes were explored, such as the horrors of the Holocaust. For example, A. M. Kleins' The Second Scroll, was well received.