What Trump is proposing is clearly illegal under the DoD Law of War manual.
For instance, I was involved in targeting during the Persian Gulf War. We did bomb bridges and power plants, but those were strategically chosen. The bridges we bombed were those we knew from intelligence carried military communication cables. When we bombed a power plant, we first selected those we knew directly fed military installations, and we precisely struck portions of power plants only to reduce their capacity. Any more than that is a war crime.
My hope is that the senior military officials communicate a determination to the White House that they will not to go down as war criminals. There's no statute of limitation for war crimes, and they want their careers to last beyond Trump's term in office (maybe even beyond his lifetime).
Yes, the president could give them all blanket pardons before they're even prosecuted, but they'd have to stay in the US for the rest of their lives. Becoming "undeployable" is a death sentence for a military career.