I was thinking of a story I wanted to write, but I had a legal question.
It involves a war story, and a group of around 12 soldiers. Members of a regular armed force, with the authority of the head of state legally behind them, have taken a position in an armed conflict and have in the process captured some of their enemies, who haven't actually said they gave up nor did they raise their hands but were physically overpowered. Those enemies are not part of another country, but are members of a designated terrorist group – some of their members having been convicted of it, even having had their death sentences commuted years before.
The allies of those enemies are coming back and are clearly going to take the position back, imminently, within a minute or two, and there is only room for the soldiers to retreat back to safety.
One of the soldiers asks their commander if it would be a good idea to kill the ones they captured before they leave, given that they can't take the captives (who are about to rejoin their enemies) with them. Does the commander have the right to do this?
I am planning to add a dialogue with one of the soldiers who is brilliant and is legally trained, and I want to know what this advisor should legally say in accordance with the laws of war in effect today.
If it matters which conventions have been ratified, I would like to know which ones are relevant.
The municipal laws of a country aren't important; the story will invent that later. I am interested in what the international treaties would say on this.
OP comments added for clarification and posterity:
This is more like a far future event, hundreds of years from now, with a long era of peace, and this is basically completely ad hoc to create a militia, as for why nobody remembers precisely what is in these conventions, not even the king. Don't worry that this is a real scenario.
This isn't really like an army of any real country. The idea I had was futuristic and a little magical in the literal sense, although then again, Thor is noted for having said in the Avengers movies with advanced technology being indistinguishable from magic. It would be centuries, maybe over a thousand years, into the future where war has been essentially absolute for that time. This scenario involves no vehicles, little prep (like maybe 5 minutes max for capturing and being forced out), no fortifications, no evac this way, just running or flying the way that Iron Man flies.