Stieff joined the Wehrmacht General Staff in 1938, serving in the Organisationsabteilung (coordination department) under Major Adolf Heusinger.

Recognized for his excellent organizational skills, Stieff in October 1942 was appointed Chief of Organisation at OKH, despite Hitler's strong personal dislike.

When in Warsaw in November 1939, he wrote letters to his wife expressing his disgust for and despair over Hitler's conduct of the war and the atrocities committed in occupied Poland.

He provided the explosives for von dem Bussche's canceled assassination attempt on Hitler at the Wolfsschanze (Wolf's Lair) in November.

In the morning of 20 July, Stieff flew with Stauffenberg and Lieutenant Werner von Haeften in the Heinkel He 111 plane provided by General Eduard Wagner from Berlin to the Wolfsschanze.