In February 1915, after World War I broke out in France, he was interned in Châteauroux at Bitray, a camp reserved for foreign civilians and located in the premises of the insane asylum of the city.
Beginning in 1924, Groethuysen participated each year in the Pontigny Decades held by Paul Desjardins at the Pontigny Abbey where Groethuysen had the opportunity to interact with French intellectuals such as Louis Aragon, Julien Benda, Léon Brunschvicg, François Mauriac, André Maurois, Gabriel Marcel, Roger Martin du Gard and Philippe Soupault.
In 1926 he collaborated with Jean Paulhan on the Library of Ideas, a collection published by in Éditions Gallimard that would soon become famous.
Groethuysen's openness of spirit, his appetite for knowledge and his generosity make him one of the great European intellectuals of the first half of the twentieth century.
He contributed to the introduction of Kafka in France, writing a preface to Alexander Vialatte's 1946 translation of The Trial.
[5] Groethuysen's works focused on the philosophy and politics of the eighteenth century, particularly Montesquieu, Rousseau and the French Revolution.