In the Spanish Civil War he acted as Chief Medical Officer to the 12th International Brigade, where he became close friends with the authors Ernest Hemingway and Gustav Regler (the latter was the 12th's political commissar). He was killed on the Cuenca Front at the foot of the Pyrenees, machine-gunned from a Nationalist plane; the pilot later claimed to have mistaken him for the leader of the Brigade. Hemingway directed that the royalties from his book The Spanish Earth should be paid to Werner's widow. He is immortalised as "Werner" in Regler's iconic autobiographical novel The Great Crusade (Der große Beispiel).