[4][5] Dismissed from his museum posts by the Vichy government, he continued his research privately and published three volumes on China and the Mongols during the war.

The British historian Christopher Tyerman points out that upon publication, Grousset's History of the Crusades came under criticism, on the one hand for not analyzing the political system of what Grousset considered to be a French state in the Levant and on the other hand for exaggerating or misrepresenting the cultural sympathy between overseas communities.

In 2001, fr:Joël Gourdon wrote: "René Grousset produced a work entirely dedicated to France's colonial role.

He sees in the colonial adventure the admirable synthesis of the most sacred values for him: Christianity, the fatherland and the State, even republican.

"[10] For fr:Vadime Elisseeff, who succeeded him as director of the Cernuschi Museum, Grousset is "the last of the great classics, those for whom the “sense of history” was more a matter of psychology of beings than of the material conditions of existence, whose physical and moral impact on the lives of individuals had not yet been emphasized by the sciences.