Related Rule
Practice Relating to Rule 1. The Principle of Distinction between Civilians and Combatants
Section B. Attacks against combatants
Mexico’s Army and Air Force Manual (2009) states:
The underlying idea of this body of law [i.e. IHL] is to humanize war. The three main principles established to this end … [include]:
A. military operations may only be directed against combatants. 
Mexico, Manual de Derecho Internacional Humanitario para el Ejército y la Fuerza Área Mexicanos, Ministry of National Defence, June 2009, § 372(A).
Mexico’s IHL Guidelines (2009), in a section titled “Basic rules of conduct in armed conflict”, states: “Fight only enemy combatants”. 
Mexico, Cartilla de Derecho Internacional Humanitario, Ministry of National Defence, 2009, § 14(d).
At the CDDH, Mexico stated that it believed Article 47 of the draft Additional Protocol I (now Article 52) to be so essential that it “cannot be the subject of any reservations whatsoever since these would be inconsistent with the aim and purpose of Protocol I and undermine its basis”. 
Mexico, Statement at the CDDH, Official Records, Vol. VI, CDDH/SR.41, 26 May 1977, p. 193.