Related Rule
Practice Relating to Rule 46. Orders or Threats That No Quarter Will Be Given
Germany’s Military Manual (1992) states: “It is prohibited to order that there shall be no survivors. It is also prohibited to threaten an adversary therewith or to conduct military operations on this basis.” 
Germany, Humanitarian Law in Armed Conflicts – Manual, DSK VV207320067, edited by The Federal Ministry of Defence of the Federal Republic of Germany, VR II 3, August 1992, English translation of ZDv 15/2, Humanitäres Völkerrecht in bewaffneten KonfliktenHandbuch, August 1992, § 450.
Germany’s Law Introducing the International Crimes Code (2002) punishes anyone who, in connection with an international or a non-international armed conflict, “orders or threatens, as a commander, that no quarter will be given”. 
Germany, Law Introducing the International Crimes Code, 2002, Article 1, § 11(1)(6).
In the Stenger and Cruisus case before Germany’s Leipzig Court after the First World War, one of the accused was charged with having issued an order that:
No prisoners are to be taken from to-day onwards; all prisoners, wounded or not, are to be killed
All the prisoners are to be massacred; the wounded, armed or not, are to be massacred; even men captured in large organised units are to be massacred. No enemy must remain alive behind us.
The other accused was charged with having passed on the order. The first accused was acquitted because it could not be proved that he had actually given the order in question. As to the second accused, the Court held:
[He] acted in the mistaken idea that General Stenger, at the time of the discussion near the chapel, issued the order to shoot the wounded. He was not conscious of the illegality of such an order, and therefore considered that he might pass on the supposed order to his company, and indeed must do so.
So pronounced a misconception of the real facts seems only comprehensible in view of the mental condition of the accused … But this merely explains the error of the accused; it does not excuse it … Had he applied the attention which was to be expected from him, what was immediately clear to many of his men would not have escaped him, namely, that the indiscriminate killing of all wounded was a monstrous war measure, in no way to be justified. 
Germany, Leipzig Court, Stenger and Cruisus case, Judgment, 1921.