Related Rule
Practice Relating to Rule 4. Definition of Armed Forces
Section B. Incorporation of paramilitary or armed law enforcement agencies into armed forces
Spain’s LOAC Manual (1996) states that members of the Guardia Civil are lawful combatants. 
Spain, Orientaciones. El Derecho de los Conflictos Armados, Publicación OR7-004, 2 Tomos, aprobado por el Estado Mayor del Ejército, División de Operaciones, 18 March 1996, Vol. I, § 1.3.a.(1).
Pursuant to Spain’s Military Criminal Code (1985), the Guardia Civil is an armed military body that exclusively falls under the responsibility of the Ministry of Defence, in times of siege warfare or when called upon to carry out missions of a military nature. 
Spain, Military Criminal Code, 1985, Article 9.