"Measures of reprisal against prisoners of war are prohibited. (GPW, art. 13.)"
Not to be negatively argumentive here, but isn't the Geneva Convention a contract between nations? If one warring party breaks the contract by breaking the rules, the contract is nullified and is no longer in effect between those nations engaged in battle, just like any other contract would be nullified by both parties the moment one party violates the terms of the contract.
Isn't this the case here?