During the Second World War Kielmansegg served in various divisions, staff regiments and fronts in Poland, France and Russia. From 1942 to 1944 he served as General Staff officer to the High Command of the Wehrmacht (OKW). On 1 May 1944, he was promoted to colonel. Several months later, he was arrested by the Gestapo for being a co-conspirator of the 20 July plot, but released on 23 October 1944 for a lack of evidence. He later said: