He was the author of the Cornides Report, which contains his account of the extermination of Jews at Belzec during the Holocaust.

In December 1946 Cornides became the founder of Europa-Archiv (renamed Internationale Politik in 1995), the first post-war magazine in Allied-occupied Germany.

In 1955 he was instrumental along with Theodor Steltzer, Minister-President of Schleswig Holstein and former member of the dissident Kreisau Circle, in founding the German Council on Foreign Relations (Deutsche Gesellschaft für Auswärtige Politik, DGAP).

[1] On 30 August 1942, during the occupation of Poland by Nazi Germany, Cornides was in Rzeszów (renamed Reichshof), on his way to the city of Chełm (Cholm) by train.

He wrote what a German railway policeman told him, that the area would soon be free of Jews (Judenfrei), since every day freight trains packed with Jews from the Generalgouvernement passed through the railway yard, and come back in the evening empty and swept clean.

[3] They were published in July 1959 by historian Hans Rothfels in the German quarterly Journal of Contemporary History (Vierteljahrshefte für Zeitgeschichte).

[1] By that time, the much more revealing Gerstein Report which featured shocking details about the extermination process at Belzec was already well known in Germany.

He arrived at Rawa Ruska junction around noon on 31 August 1942 and made further entries in his journal later that day.

"[6][7]Cornides lounged at the Deutsches Haus in Rawa Ruska before he boarded the connecting train to Chełm the same evening.

On the doors had been written in chalk: '60', '70', once '90', occasionally '40' – obviously the number of Jews inside the cattle cars.

[2]In his train compartment, Cornides talked to a German woman who had witnessed the round-up of Jews at Chełm and the shooting of those who tried to escape.

The railway policeman said: "In the railway documents these trains run under the name of resettlement transports," and added that after the murder of Reinhard Heydrich by Czech resistance members, several trains filled with Czech Jews had passed through.

One track branched off from the main line, the other ran over a turntable from the camp to a row of sheds some 250 meters away.

[2]In his typewritten pages, Cornides also summarized conversations with other Germans he met during his stopover in the Deutsches Haus at Rawa Ruska, as well as statements he remembered from Chełm upon his arrival there.

A policeman in the townhall restaurant at Cholm (Chełm) on 1 September 1942 said: "The policemen who guard the Jewish transports are not allowed inside the camp, only the SS and the Ukrainian Sonderdienst – a police formation consisting of Ukrainian auxiliaries – do so [see Trawniki men for more historical background].