相关规则
Spain
Practice Relating to Rule 145. Reprisals
Section B. Reasonable notice and measure of last resort
Spain’s LOAC Manual (1996), in the chapter dealing with the exercise of command and its restrictions with regard to reprisals, states: “Reprisals must be the ultimate resort to re-establish respect for the law of war and may not lose sight of this aim.” 
Spain, Orientaciones. El Derecho de los Conflictos Armados, Publicación OR7-004, 2 Tomos, aprobado por el Estado Mayor del Ejército, Division de Operaciones, 18 March 1996, Vol. I, § 2.3.b.(6).
In the chapter dealing with methods of combat, the manual lists among the conditions which must be fulfilled for the lawful taking of reprisals that “they must be a last resort to re-establish respect for the laws of war”; that the State which takes reprisals “has unsuccessfully tried to make the enemy respect the law of war or that such attempts would be of no use”; and that the enemy has been formally warned of the measure that would be taken if it failed to comply with or repeated its violations of the law of war. 
Spain, Orientaciones. El Derecho de los Conflictos Armados, Publicación OR7-004, 2 Tomos, aprobado por el Estado Mayor del Ejército, Division de Operaciones, 18 March 1996, Vol. I, § 3.3.c.(5)(a).
Spain’s LOAC Manual (2007) states: “Reprisals can only be used as a last resort to make the enemy comply with the law of armed conflict”. 
Spain, Orientaciones. El Derecho de los Conflictos Armados, Tomo 1, Publicación OR7–004, (Edición Segunda), Mando de Adiestramiento y Doctrina, Dirección de Doctrina, Orgánica y Materiales, 2 November 2007, § 2.3.b.(6).