After the 1941 German invasion of Russia and signing the Sikorski-Mayski Agreement, Czapski joined the Polish II Corps under the command of General Anders. Between 1941 and 1942, Czapski was tasked with investigating the disappearance of Poles who had been in the captivity of the NKVD and subsequently massacred. He never received any satisfactory answers as to the fate of these men, but wrote about his experiences in two books, "Reminiscences of Starobyelsk" (1944) and "The Inhuman Land" (1949). During that period Czapski also met Aleksey Nikolayevich Tolstoy and Anna Akhmatova who is said to have dedicated one of her poems to him.