War crimes are not allowed under international law as part of a tit-for-tat.

There is some nuance here, where some war crimes could make what would normally be a warcrime by the other side not a war crime. For instance, it is a war crime to attack medics, and it is a warcrime for combatants to pose as medics. However, if a country commits the warcrime of having combatants pose as medics, it is no longer a warcrime to attack that countries medics.