Related Rule
Canada
Practice Relating to Rule 145. Reprisals
Section B. Reasonable notice and measure of last resort
Canada’s LOAC Manual (1999) provides:
To qualify as a reprisal, an act must satisfy the following conditions:
c. There must be reasonable notice that reprisals will be taken. What degree of notice is required will depend upon the particular circumstances of each case. Notice is normally given after the violation but may, in appropriate circumstances, predate the violation. An example of notice is an appeal to the transgressor to cease its offending conduct and punish those responsible. Thus, such an appeal may serve both as a plea for compliance and a notice to the adversary that reprisals will be undertaken;
d. The victim of a violation must first exhaust other reasonable means of securing compliance in order to justify taking a reprisal. 
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”:
1. In the event of serious or persistent breaches of the LOAC it may be necessary for the adverse party to resort to a reprisal in an attempt to terminate the illegality. [A] reprisal is an illegal act resorted to after the adverse party has performed illegal acts and has refused to stop after being called upon to do so.
6. To qualify as a reprisal, an act must satisfy the following conditions:
c. There must be reasonable notice that reprisals will be taken. What degree of notice is required will depend upon the particular circumstances of each case. Notice is normally given after the violation but may, in appropriate circumstances, predate the violation. An example of notice is an appeal to the transgressor to cease its offending conduct and punish those responsible. Thus, such an appeal may serve both as a plea for compliance and a notice to the adversary that reprisals will be undertaken;
d. The victim of a violation must first exhaust other reasonable means of securing compliance in order to justify taking a reprisal. 
Canada, The Law of Armed Conflict at the Operational and Tactical Levels, Office of the Judge Advocate General, 13 August 2001, § 1507.1 and 6.c–d.