1. In an earlier post, I mentioned a paper by Jack Reynolds about the impact on philosophy of WWI. It turns out that Reynolds' paper is on-line at Academia.
2. Among philosophical schools of thought, perhaps neo-Kantianism suffered the most as a result of WWI. Prominent neo-Kantians, such as Alois Riehl and Wilhelm Windelband, signed the infamous 'Manifesto of the Ninety-Three' (1914), in which professors defended Germany's war effort (inc. the invasion of Belgium). Eventually, approximately four thousand German academics endorsed the claims of the manifesto, including many more philosophers. I'm not aware of any neo-Kantians who opposed the War. This might be one reason why the neo-Kantians didn't appear in histories of analytic philosophy until recently: after the War, anglo-philosophers didn't want to read them.
3. To get a sense of how philosophers (and some religious thinkers) in the UK and its allied nations viewed the War during its early stages, look at volume 13 (Oct. 1914-July 1915) of the Hibbert Journal, which includes papers by Norman Kemp Smith, G. Dawes Hicks ('German Philosophy and the Present Crisis'), Henri Bergson, Evelyn Underhill, Count Hermann Keyserling, and E. F. Carritt. Nietzsche -- er, straw-Nietzsche -- takes a beating.
4. One of the most Hegelian British philosophers of that era was Sir James Black Baillie, who translated Hegel's Phenomenology. In A Hundred Years of British Philosophy (1938, pp. 317-8), Rudolf Metz wrote that the War prompted Baillie to abandon idealism. Metz based this claim largely on Baillie's Studies in Human Nature. Recently, Peter Hoerres has repeated Metz's claim. (Hoerres, 'Idealism as Transnational War Philosophy, 1914-1918', in Anglo-German Scholarly Networks in the Long Nineteenth Century, ed. Ellis and Kirchberger [Brill, 2014]) However, Metz's claim is disputed on p. 51 of the Dictionary of Twentieth-Century British Philosophers.
More about Baillie: Too old to join the army in WWI, Baillie served in the Admiralty's Intelligence Division for a couple of years and also was on a London conscientious objector tribunal. After the War, Baillie held several administrative positions. He was a labour arbitrator and lawyer and was knighted in 1931. He was Vice-Chancellor of the University of Leeds. According to his entry in the Oxford Dictionary of National Biography, 'Baillie is presented as Sir John Evans in Michael Innes's novel The Weight of the Evidence (1944).' For more about Baillie, see David Charlston's dissertation, Hegel's Phenomenology in Translation: A comparative analysis of translatorial hexis. (2012, University of Manchester)
5. In response to the vilification of German philosophy, J. H. Muirhead defended some German philosophers (esp. Hegel) in German Philosophy in Relation to the War.
6. Outside the UK, other philosophers who served on the WWI front lines include: Eugen Rosenstock-Huessy, Franz Rosenzweig, Paul Tillich, Ludwig Wittgenstein, Karl Löwith, Rudolf Carnap, Hans Reichenbach, Alfred Schutz, Michael Polanyi, Gaston Bachelard, André Maurois, Alexander Koyré, Alain.