Psalms 110 is obviously prophetic, and it applies to the soldiers involved. Was he under the command of officers and a government that was doing God's bidding or not?
Prophetic, yes.
It has absolutely nothing to do with soldiers fighting at anyones command.
The subject of the psalm is Jesus. He is the one that will do these things.
As for soldiers dying in defense of king and country, Jesus plainly said there was but one way to Heaven and He is it.