Full Idea
It is not unlawful to put a city to sack, if it is necessary for the conduct of the war …as a spur to the courage of the troops.
Gist of Idea
Sacking a city is lawful if it motivates the attacking troops
Source
Francis de Vitoria (On the Law of War [1525], p.184), quoted by Michael Walzer - Just and Unjust Wars 08
Book Reference
Walzer,Michael: 'Just and Unjust Wars' [Penguin 1984], p.134
A Reaction
Hideous. Presumably this would include raping the women. Could you motivate a football team in a similar way? Or to get your children to pass exams?