Trump has pardoned several people who attempted to breach the capital and murder elected representatives to stop the ratification of a democratic presidential election.
All he needs is a single paper that says "All members of the military following the orders of President Trump are pardoned for all crimes past and future related to said orders" and - boom - accountability gone!
For now. But there is no reason why a future government of the USA would not see that in an entirely different light. After all, those that went to trial after WWII also argued that their actions were legal and it turned out not to be the case.
There is no legal mechanism for unpardoning someone, the constitution only allows it to go one way. It would require an amendment, and we can't even pass laws with this legislature.
And for good reason, mind you. Same reason as dismissal without prejudice gets some flak as easily abused for corrupt leverage.
A future government is not necessarily bound to the one the preceded it depending on the way the one government changed into the other. And the way things are going right now there is a non-zero chance that the USA will see a reboot of its system of government, if it doesn't actually fall apart into multiple different countries.
All he needs is a single paper that says "All members of the military following the orders of President Trump are pardoned for all crimes past and future related to said orders" and - boom - accountability gone!