Stuart Ford’s article, Has President Trump Committed a War Crime by Pardoning War Criminals? 35 AM. U. INT’L. L. REV. 757 (2020) seeks to answer that from an international law perspective through the lens of command responsibility.
Command responsibility generally has two sides–the duty to prevent and the duty to punish.
One interesting question is whether clemency after trial would be a war crime under the author’s theory. For example, Behenna served a significant period of confinement and there were legitimate concerns about some Brady-plus issues. It does seem the author considers a preemptive pardon a potential war crime–pretrial for example.