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Switzerland
Practice Relating to Rule 53. Starvation as a Method of Warfare
Section A. General
Switzerland’s Basic Military Manual (1987) states with regard to civilians who are in the power of the troops at the time of combat: “It is prohibited to starve the civilian population by removing or rendering supplies useless, or by impeding relief actions in favour of the population in need.” 
Switzerland, Lois et coutumes de la guerre (Extrait et commentaire), Règlement 51.7/II f, Armée Suisse, 1987, Article 147(b).
Switzerland’s Regulation on Legal Bases for Conduct during an Engagement (2005) states:
14.4 Objects indispensable to the survival of the civilian population
216 Starvation of the population as a method of warfare is prohibited.
15.2 Prohibited methods of warfare
226 Starvation of and threats against the civilian population … are prohibited in any place and at any time. 
Switzerland, Bases légales du comportement à l’engagement (BCE), Règlement 51.007/IVf, Swiss Army, issued based on Article 10 of the Ordinance on the Organization of the Federal Department for Defence, Civil Protection and Sports of 7 March 2003, entry into force on 1 July 2005, §§ 216 and 226.
Switzerland’s ABC of International Humanitarian Law (2009) states: “Even in war not everything is allowed. Various means and methods are prohibited, including … starvation”. 
Switzerland, Federal Department of Foreign Affairs, ABC of International Humanitarian Law, 2009, p. 29.