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Canada
Practice Relating to Rule 36. Demilitarized Zones
Section B. Attacks on demilitarized zones
Canada’s LOAC Manual (1999) states: “It is prohibited for parties to a conflict to conduct military operations in or to attack an area that they have agreed to treat as a demilitarized zone.” 
Canada, The Law of Armed Conflict at the Operational and Tactical Level, Office of the Judge Advocate General, 1999, p. 4-11, § 115.
The manual further states that an area loses its status as a demilitarized zone if used “for purposes related to the conduct of military operations where it has agreed not to do so”. 
Canada, The Law of Armed Conflict at the Operational and Tactical Level, Office of the Judge Advocate General, 1999, p. 4-11, § 118(b).
The manual considers that “making … demilitarized zones the object of attack” constitutes a grave breach of the 1977 Additional Protocol I. 
Canada, The Law of Armed Conflict at the Operational and Tactical Level, Office of the Judge Advocate General, 1999, p. 16-3, § 16(d).
Canada’s LOAC Manual (2001) states in its chapter on targeting:
1. It is prohibited for parties to a conflict to conduct military operations in or to attack an area that they have agreed to treat as a demilitarized zone.
4. An area loses its status as a demilitarized zone where:
b. a party uses the demilitarized zone for purposes related to the conduct of military operations where it has agreed not to do so. 
Canada, The Law of Armed Conflict at the Operational and Tactical Levels, Office of the Judge Advocate General, 13 August 2001, § 457.1 and 4.b.
In its chapter on “War crimes, individual criminal liability and command responsibility”, the manual provides that “making … demilitarized zones the object of attack” constitutes a grave breach of the 1977 Additional Protocol I. 
Canada, The Law of Armed Conflict at the Operational and Tactical Levels, Office of the Judge Advocate General, 13 August 2001, § 1608.2.d.
Canada’s Geneva Conventions Act (1985), as amended in 2007, provides: “Every person who, whether within or outside Canada, commits a grave breach [of the 1977 Additional Protocol I] … is guilty of an indictable offence.” 
Canada, Geneva Conventions Act, 1985, as amended in 2007, Section 3(1).