This war has indeed wrought great havoc in scholarship. (Edward P. Buffet, Monist 26, no. 2 [April, 1916], 'Karl Eugen Neumann', p. 319)In addition to Étienne Gilson, another French philosopher who served in WWI is Émile Bréhier. Although he was 40 years old in 1916, he was at the Battle of Verdun, as a result of which his left arm was amputated (in October, 1916). Martial Gueroult also was in the army, as was the historian Marc Bloch.
On the German side, there is Heinrich Friedemann, who had studied philosophy at Marburg with Paul Natorp and belonged to one of Stefan George's circles. Friedemann had published a book on Plato (Platon; seine Gestalt, 1914) and was, when the War began, habilitating at Heidelberg with Friedrich Gundolf. On Feb. 22, 1915, Friedemann was killed in action.
Another member of George's circle who was killed in the War is Norbert von Hellingrath. He was a Munich philologist who is known for having re-discovered Hölderlin's poetry.
From Ernst Robert Curtius's letter to Friedrich Gundolf (November, 1914):
What is horrifying about modern war is that human beings do not fight against other human beings, but against gruesome impersonal machines. Land mines, machine guns, artillery fire: that is the anonymous horror on which every idealistic perception of the war must founder. (quoted from Robert E. Norton, Secret Germany: Stefan George and His Circle, [Cornell University Press, 2002], p. 530)There was also Wilhelm Metzger, who taught at Leipzig. He was a prominent Schelling scholar and had published Die Epochen der Schellingschen Philosophie von 1795 bis 1802, Ein problemgeschichtlicher Versuch. He was drafted into the army and sent to the western front. You will have to excuse my extremely weak German-language abilities, but if I interpret correctly a passage from Peter Hoerres' book (Krieg der Philosophen: die deutsche und britische Philosophie im Ersten Weltkrieg, 2004), Metzger died in 1916 of cancer.
Outside of philosophy but doing work of interest in the philosophy of art, there was Fritz Burger, an art history professor in Munich who was killed on May 22, 1916. Born in 1877, Burger was almost as old as Bréhier.
Robert Staiger, a privatdozent in art history at Göttingen and Felix Klein's son-in-law, was killed in action in August, 1914. Another Göttingen privatdozent who was killed in the War, and who was close to Klein, was the mathematician, Wilhelm Behrens.
I learned of Behrens from this incredible source: Placing World War I in the History of Mathematics, by David Aubin and Catherine Goldstein (2013). See also the collection edited by Aubin and Goldstein: The War of Guns and Mathematics, ed. David Aubin and Catherine Goldstein (American Mathematical Society, 2014)
Here's a quotation from Arthur Bauer's piece in the July, 1916 issue of Revue Philosophiques de la France et de l'Étranger. Treat my following attempt at translation more as a paraphrase. According to Bauer, in extended periods of peace, for philosophers who do not attend sufficiently to history,
Humanity is in a state of constant progress and moves ... towards the ultimate end of its evolution: an altruism so natural and so powerful that everyone, forgetting his own interests, would serve his neighbor from inexhaustible reserves of charity. ... The terrible shock of current events wakes from their dream the most obstinate sleepers. By the light of howitzers that ... destroy whole battalions, illusions vanish, and reality appears in a relief so striking that no one can mistake it. (Bauer, 'Le role de la Fôrce' Revue Philosophiques de la France et de l'Étranger 82 [1916], p. 44)In the Austro-Hungarian forces, in addition to Wittgenstein, philosopher Ernst Mally was in the front lines. Vienna-Circle mathematician Hans Hahn was wounded in the Isonzo region.
There was also a philosophy student and poet, Franz Janowitz, who was killed in 1917 in the Isonzo area. Janowitz, a friend of Franz Werfel's, was from Prague. His poetry was admired by Max Brod and Karl Kraus. He was studying philosophy in Vienna when the War began. One of his older brothers, Hans Janowitz (also a war vet), co-wrote the script for The Cabinet of Dr. Caligari.
Jerzy Żuławski
Photo source.Jerzy Żuławski joined the Polish forces that fought on the Austro-Hungarian side. Żuławski had studied philosophy with Richard Avenarius in Switzerland. The product of that work was a dissertation called The Problem of Causality in Spinoza, which was the basis for a popular-philosophy book on Spinoza that Żuławski later published in Polish (Benedykt Spinoza: człowiek i dzieło, 1902). Żuławski translated several works of German philosophy into Polish. He is now known chiefly for his science-fiction books, especially his Lunar Trilogy (or Moon Trilogy). Stanisław Lem read these books when he was young and continued to admire the first book of the trilogy (The Silver Globe) later in life. Here's a 1985 item that Lem wrote about Żuławski's work. A film version of The Silver Globe was made by Żuławski's grand-nephew Andrzej Żuławski in 1976. The Lunar Trilogy sounds interesting. One of its themes was the transposition of events from the remote past into the language of mythology; the distant historical events in the novels concern the formation of a colony on the moon. While it has been translated into several languages, The Silver Globe hasn't been rendered in English. In WWI, Jerzy Żuławski worked mainly behind the battle front. He died in 1915 from an illness that he caught while visiting the front line.
From F. C. S. Schiller's review of John Dewey's book German Philosophy and Politics:
[The War] revealed that the actual world was a very different thing from the cosmic order they [philosophers] had constructed in their minds....Even though the rational order of human affairs was shattered before their eyes and the belief that thought controls man's feelings and determines his acts should have been among the first of the illusions swept away in the wreckage of the war, they insisted on finding ideal reasons to which to attribute the catastrophe.' (Schiller, Mind 25, no. 98 (Apr., 1916): 250)Other philosophy students who served in the War and who achieved some distinction as novelists or poets include:
Miloš Crnjanski -- a Serb in the Austro-Hungarian army who wrote the The Journal of Carnojevic, which was recently produced as a play in Belgrade;
Camil Petrescu -- a Romanian who was wounded and taken prisoner by the Austro-Hungarians;
Milutin Bojić -- a poet in the Serbian army who died in May, 1917;
Sima Pandurović -- another poet in the Serbian army, Pandurović was taken prisoner by the Austro-Hungarians and survived the War.Finally, here's an article by Nil Santiáñez (of St. Louis University) comparing Ernst Jünger's WWI writing with that of communist author Adam Scharrer; the article is called 'Ernst Jünger and Adam Scharrer: Two Global Views of the Great War'.
