Related Rule
Germany
Practice Relating to Rule 36. Demilitarized Zones
Germany’s Military Manual (1992) states:
The prerequisites for establishing [a demilitarized zone] are equal to those applying to non-defended localities (Article 59 para. 2, 60 para. 3 of the 1977 Additional Protocol I). Demilitarized zones are created by an agreement concluded between the parties to the conflict either in peacetime or in case of conflict. It is prohibited for each party to the conflict to attack or occupy such zones (Article 60 para 1 of the 1977 Additional Protocol I). 
Germany, Humanitarian Law in Armed Conflicts – Manual, DSK VV207320067, edited by The Federal Ministry of Defence of the Federal Republic of Germany, VR II 3, August 1992, English translation of ZDv 15/2, Humanitäres Völkerrecht in bewaffneten KonfliktenHandbuch, August 1992, § 461.
Germany’s Military Manual (2001) provides that “it is prohibited for each party to the conflict to attack or occupy [demilitarized] zones”. 
Germany, Humanitarian Law in Armed Conflicts – Manual, DSK VV207320067, edited by The Federal Ministry of Defence of the Federal Republic of Germany, VR II 3, August 1992, English translation of ZDv 15/2, Humanitäres Völkerrecht in bewaffneten KonfliktenHandbuch, August 1992, § 461.
The manual points out that, if one of the parties to the conflict breaches the provisions concerning the conditions for the establishment of demilitarized zones, the zone in question will lose its special protection”. 
Germany, Humanitarian Law in Armed Conflicts – Manual, DSK VV207320067, edited by The Federal Ministry of Defence of the Federal Republic of Germany, VR II 3, August 1992, English translation of ZDv 15/2, Humanitäres Völkerrecht in bewaffneten KonfliktenHandbuch, August 1992, § 462.
The manual further provides that grave breaches of IHL are in particular “launching attacks against … demilitarized zones”. 
Germany, Humanitarian Law in Armed Conflicts – Manual, DSK VV207320067, edited by The Federal Ministry of Defence of the Federal Republic of Germany, VR II 3, August 1992, English translation of ZDv 15/2, Humanitäres Völkerrecht in bewaffneten KonfliktenHandbuch, August 1992, § 1209.
Germany’s Law Introducing the International Crimes Code (2002) provides for the punishment of anyone who, “in connection with an international armed conflict or with an armed conflict not of an international character, … directs an attack by military means against … demilitarized zones”. 
Germany, Law Introducing the International Crimes Code, 2002, Article 1, § 11(1)(2).