In Europe, the First World War 1914-18, also called the Great War, was an extremely traumatic event, that stopped an era of well-being and progress (often referred to as La belle époque).
The European war 1914-18 involved all social components of the major European nations, with intellectuals and students in the forefront.
The consequences were disastrous in terms of loss of human lives and
careers of excellent scholars cut short. A drama that changed the destiny of a generation of scientists in many countries, the scientific community itself, and the course of scientific research.
Among the scientists, many mathematicians of many European countries died in the battlefield and in the trenches during the Great War.
I mention here, in particular, the tragedy of what has been called the incomprensible écatombe
in France. In particular, 800 students of the École Normale Supérieure of Paris took part in the war, and 239 of them lost their lives. Among them, also many mathematicians (see $[1]$).
The war left a deep wound and a traumatic memory in the country, which influenced also the Bourbaki group: they described the post-war period in France as a “mathematic desert”, and they attributed the causes to the loss of a generation, decimated by the war.
According to some historian of mathematics, the trauma of the war influenced the abstract nature of the mathematics of the Bourbaki group. The historian of mathematics David Aubin writes:
Dans les années 1930, le groupe de mathématiciens Bourbaki a promu une
vision abstraite et universelle de sa discipline en rupture avec celle
de ses prédécesseurs. Le sacrifice de nombreux jeunes mathématiciens
durant la Première Guerre mondiale a sans doute fortement contribué à
ce revirement.
[During the years 1930, the Bourbaki group of mathematicians has
prompted an abstract and universal vision of their subject, in rupture
with the one of their predecessors. The sacrifice of many young
mathematicians during the First World War has undoubtedly contributed
to this reversal] ( $[2]$, p.1, my transl.)
And in his book L’Élite sous la mitraille $[1]$ David Aubin writes:
Au course des semaines le plus sanglantes de l’histoire de France,
entre le 22 août et le 20 octobre 1914, douze mathématiciens élèves
de l’École Normale supérieure de la rue d’Ulm, à Paris, tous agrégés
de mathématiques et, pour la plupart, auteurs de travaux remarqués,
publiés et recencés dans le repertoire de reference, le “Jahrbuch
über die Fortschritte der Mathematik” (JFM), sont tués sur le champs
de bataille du Nord et de l’Est. Dans les années qui suivent, dix
autres jeunes mathematicians connaîtront le même sort.
During the bloodiest weeks of the history of France, between the 22th
of August and the 20th of October 1914, twelve mathematicians, all
former students of the École Normale Supérieure de la rue d’Ulm, in
Paris, all of them agrégés of mathematics and, for the most part,
authors of works reported, published and reviewed in the bibliographic
repertory, the “Jahrbuch über die Fortschritte der Mathematik” (JFM),
are killed on the battlefield of the North and the East. In the
following years, ten other young mathematicians will
suffer the same fate. (p. 7, my transl. )
In 1923, built thanks to a subscription, the Monument aux morts de l’École Normale Superiéure [Monument to the Victims of the École Normale Superiéure] in Rue de l’Ulm, by the sculptor Paul Landowski, was inaugurated.
Out of $239$ normaliens whose name appears on the monument to the victims, $22$ are agrégés of mathematics.
Below the Tableau that gives the list of those mathematicians who died in the First World War: as the table shows, fourteen of them died in 1914, four in 1915, three in 1916 and another, Paul Viple, some days after the Armistice, because of the injuries sustained on the 25th of October 1918.
(source $[1]$, p. 10)

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We can notice the name of René Gateaux, whose name is known for the Gateux derivative, and whose works on functional analysis impressed the famous Italian mathematician Vito Volterra.
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Below an image of the Monument to the Victims of the École Normal Supérieure:

References
$[1]$ Aubin, David, L’élite sous la mitraille. Les normaliens, les mathématiques et la Grande Guerre, 1900-1925, Editions Rue d’Ulm/Presse de l’École normale supériueure, 2018.
$[2]$ Aubin, David, ‘Des mathématiciens sous la Grande Guerre’,
https://www.pourlascience.fr/sd/histoire-sciences/des-mathematiciens-sous-la-grande-guerre-20451.php