1. First, in a recent paper, Stewart Candlish asks, 'How do we account for the wider decline in allegiance to idealism?' ('Philosophy and the Tide of History: Bertrand Russell's Role in the Rise of Analytic Philosophy', The Historical Turn in Analytic Philosophy, ed. Erich H. Reck [Palgrave Macmillan, 2013], p. 55) He ventures the hypothesis that idealism was a surrogate for traditional forms of religion, offering succour in the face of hard facts. Candlish adds that the ascendancy of Darwinian naturalism reduced the need for such 'consolation'. He then says,
Consolatory views began to look positively distasteful in the aftermath of the Great War. After all the mud, the gas, the relentless shelling, the blood and slaughter, idealism's central tenet of the spirituality of the universe was offensive when not merely laughable, particularly so when we remember its German origins. (Ibid.)Candlish refers to Paul Fussell's study of the disillusioning impact of WWI, in the light of which the spiritual consolations afforded by idealism and religion alike were exposed to mockery.
Candlish offers these reflections as only a 'sketch of a partial answer'. (Ibid.) I have the following reservations. First, there are forms of idealism that don't offer consolation (e.g., Schopenhauer's version). Second, most of the British, WWI veterans whom I've identified maintained their commitment to idealist or to more traditionally religious frameworks after the War. Why say that the mud, gas, shelling, and so on militated against these outlooks when most of the UK philosophers who suffered these assaults first-hand were not thus disabused? Perhaps idealism was weakened not by the adversity, itself, but by the way the broader culture remembered and interpreted the adversity. Third (and in connection with those 'German origins'): absolute idealism took root and spread in early nineteenth-century Prussia, a culture that was not unfamiliar with the ravages of war.
2. In his The Cultural Politics of Analytic Philosophy, Thomas L. Akehurst says that one effect of the War was to diminish the engagement of British philosophers with German philosophy. I haven't yet looked at Akehurst's book very closely, but he (among other things) examines the vilification of German philosophers (esp. Hegel) by L. T. Hobhouse and others in the wake of WWI, and Akehurst traces this trend in the works of various British philosophers.
3. Flipping through the 1914-19 volumes of Mind and the Aristotelian Society's Proceedings, I found relatively few references to the War. In the brief summaries of the Society's meetings, there are repeated expressions of concern about the increased cost of paper (due to the War) along with encouraging statements to the effect that the Society's membership continues to grow. In Volume 17 of the Proceedings (1916-17, pp. 256-99), there's a symposium on social reconstruction after the War. (The symposiasts were Lawrence Pearsall Jacks, George Bernard Shaw, C. Delisle Burns, and H. D. Oakley.)
In an issue of Mind, there's a short piece by one of the veterans, J. L. Stocks ('The Test of Experience', Mind 28 : 79-81). Stocks' topic is Aristotle's treatment of courage in connection with fear and 'cheer' (or confidence, to use the more common translation). Stocks says that before his own battlefield experiences, he shared in the widespread skepticism about Aristotle's talk of 'cheer'. However, adds Stock, after serving in the trenches and observing another soldier's behaviour, he now knows what Aristotle meant. Stocks writes of a kind of 'invigoration' and 'enjoyment' (p. 80) that he and his comrades sometimes felt in the heat of battle.
Unlike many (all?) of the other British philosophers who saw active duty in the War, Stocks had begun his professional academic career before the war began; he became a fellow and tutor at St. John's College (Oxford) in 1906. Stocks died on June 13, 1937 from complications of a war wound.