No quarter

A victor gives no quarter when the victor shows no clemency or mercy and refuses to spare the life in return for the surrender at discretion (unconditional surrender) of a vanquished opponent.

Under the laws of war, "....it is especially forbidden....to declare that no quarter will be given". This was established under Article 23 (d) of the 1907 Hague Convention IV - The Laws and Customs of War on Land.

Since a judgment on the law relating to war crimes and crimes against humanity at the Nuremberg Trials in October 1946, the 1907 Hague Convention, including the explicit prohibition to declare that no quarter will be given, are considered to be part of the customary laws of war and are binding on all parties in an international armed conflict.

Etymology
The term may originate from an order by the commander of a victorious army that they "will not quarter (house)" captured enemy soldiers. Therefore, none can be taken prisoner and all enemy combatants must be killed. A second derivation given equal prominence in the Oxford English Dictionary (OED) is that quarter (n.17) can mean "Relations with, or conduct towards, another" as in Shakespeare Oth. II. iii. 180, "Friends all..In Quarter, and in termes like Bride, and Groome." So "no quarter" may also mean to refuse to enter in to an agreement (relations) with an enemy attempting to surrender. The OED mentions a third possible derivation but says "The assertion of De Brieux (1672 Origines..de plusieurs façons de parler, 16) that it arose in an agreement between the Dutch and Spaniards, by which the ransom of an officer or private was to be a quarter of his pay, is at variance with the constant sense of the phrases give and receive quarter."