Belgium’s Law of War Manual (1983) states that, when recourse is made to reprisals, the following conditions must be fulfilled: “3) the damage suffered by the adversary must be proportionate to the damage that he has caused”. 

In 1967, a Belgian Senator stated with respect to bombardments of North Vietnam by the United States: “It is recognized today that [reprisals] must be proportionate to the injury suffered. In case one has not suffered any damage, as it was the case, it is incomprehensible to pretend to start a period of bombardments on North Vietnam, as reprisals for attacks on the high sea.”