He was the youngest and smallest general, and displayed outstanding organizational talent, but Hitler did not conceal his personal dislike for Stieff, calling him a "poisonous little dwarf."
Stieff wrote many letters to his wife during the war illustrating his despair with Hitler's conduct of the campaigns. He was particularly outraged at the atrocities committed in Poland.
Recruited by Henning von Tresckow, Stieff joined the active resistance against Adolf Hitler. Initially, he volunteered to kill Hitler, but later reneges, and refuses repeated requests from Colonel von Stauffenberg to lend active support to the assassination plans.
He was arrested by the Gestapo on the night of July 20-21 at the Wolf's Lair and brutally tortured. He held out for several days against all attempts to extract the names of fellow conspirators. Tried by the People's Court, he was sentenced to death on August 8, and executed the same day in Plotzensee prison.