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Canada
Practice Relating to Rule 145. Reprisals
Section C. Proportionality of reprisals
Canada’s LOAC Manual (1999) states:
[The reprisal] must be proportionate to the original wrongdoing, and must be terminated as soon as the original wrongdoer ceases the illegal actions. Proportionality is not strict, for, if the reprisal is to be effective, it may often be greater than the original wrongdoing. Nevertheless, there must be a reasonable relationship between the original wrong and the reprisal measure. 
Canada, The Law of Armed Conflict at the Operational and Tactical Level, Office of the Judge Advocate General, 1999, p. 15-2, § 14.
The manual further provides:
To qualify as a reprisal, an act must satisfy the following conditions:
f. A reprisal must be proportional to the original violation. Although a reprisal need not conform in kind to the same type of acts complained of (bombardment for bombardment, weapon for weapon) it may not significantly exceed the adversary’s violation either in violence or effect. 
Canada, The Law of Armed Conflict at the Operational and Tactical Level, Office of the Judge Advocate General, 1999, p. 15-3, § 17.
Canada’s LOAC Manual (2001) states in its chapter entitled “Preventative and enforcement measures and the role of protecting powers”:
3. Reprisal is not a retaliatory act or a simple act of vengeance. It must be proportionate to the original wrongdoing, and must be terminated as soon as the original wrongdoer ceases the illegal actions. Proportionality is not strict, for, if the reprisal is to be effective, it may often be greater than the original wrongdoing. Nevertheless, there must be a reasonable relationship between the original wrong and the reprisal measure.
6. To qualify as a reprisal, an act must satisfy the following conditions:
f. A reprisal must be proportional to the original violation. Although a reprisal need not conform in kind to the same type of acts complained of (bombardment for bombardment, weapon for weapon) it may not significantly exceed the adversary’s violation either in violence or effect. 
Canada, The Law of Armed Conflict at the Operational and Tactical Levels, Office of the Judge Advocate General, 13 August 2001, § 1507.3 and 6.f.
In 1986, in an annex to a memorandum on Canada’s attitude to possible reservations with regard to the 1977 Additional Protocol I, the Canadian Ministry of Defence noted: “The act of reprisal must be proportional to the illegal act or acts committed by the other belligerent.” 
Canada, Ministry of Defence, Memorandum on Ratification of the 1977 Additional Protocol I, Reprisals Reservation, Operational Considerations, Doc. 3440-13-2 (D Law/I), 14 March 1986, Annex A, § 1.