Would the answer be different if T were only defending himself and his family against harm, and not trying to otherwise hinder the advancing army?

Let us continue this discussion in chat.

@Someone Kind of sounds like the same thing to me.

@DKNguyen I don't think they're necessarily the same; shooting at someone because you they're trying to kill you is quite different from shooting at them because they're trying to take over your government.

@Someone That's semantics. How do you expect the advancing army to tell the difference? For that matter, how do you expect the civilian to communicate that they aren't a defence force? Ultimately the actions of the advancing army and the civilian are identical. It'd probably be possible in a world with melee weapons where you can communicate and stay beyond charging and weapon range, but not with guns.

@DKNguyen if the civilian talks at all before firing, that could make it quite clear. If T says, "You're not taking over my country as long as I have any say in it!" and fires, he's quite clearly acting as a combatant. If he says, "I'm a civilian! Don't shoot!" then fires when an enemy soldier continues threatening him, he's defending himself.

@Someone You don't really expect such a fantasy situation to play out do you? You're assuming a civilian is able to approach close enough holding a gun to actually say that, and to actually be heard while holding said gun. Guns have a longer range than words, and words also need to be believed.

@DKNguyen since this is a hypothetical, let's assume that America is the country being invaded. It's quite common here for civilians to carry guns for personal self-defense, so the fact that someone has a gun doesn't make them a combatant. The gun could be concealed, so the soldier wouldn't know he was armed until he drew the gun to fire. And I don't see anything indicating how far apart T is from the enemy soldier? He could be in his house, with an advancing soldier at his door.

@Someone "the fact that someone has a gun doesn't make them a combatant" That's a lot of faith to have as an invading soldier. Another complication is what happens when the soldier orders the civilian to put their hands behind their head a kneel and the civilian refuses? If the civilian has a concealed weapon and is defending their home, they are probably going to refuse because that defeats the purpose. How is the soldier supposed to interpret things then? If they do comply then the gun was pointless to begin with, and is just going to make things go downhill when it is found.

@DKNguyen If you go to Texas, where it's common for civilians to have guns, then it's unreasonable to assume anyone with a gun is a combatant.

@Barmar You talk about that as (presumably) an American civilian and (presumably) gun-loving bias. But invading soldiers aren't going to care very much about that, and rightly so. Their lives are at stake. Turn the tables around, would you expect an American soldier to reasonably entertain that notion when entering a foreign country where civilians have lots of guns?

@DKNguyen Well, too bad. They shouldn't have invaded in the first place.

You're essentially saying that it's OK to invade a country where gun ownership is common, because you can treat all the gun owners as combatants and it's not a violation of Geneva to shoot them.

@Barmar I'm not saying its okay. Don't put words in my mouth, but how are you supposed to do any of what you say in practice? It's basically unenforceable. Imagine if it was American soldiers entering a country where civilians had lots of guns. You wouldn't expect them to entertain that notion very much either.

@DKNguyen OK, that actually happened. American soldiers in Afghanistan had to deal with threats from locals. AFAIK they didn't routinely shoot anyone with a gun, the person had to be threatening them.

@Barmar So you're telling me that the locals could be walking around American soldiers openly holding rifles and they wouldn't have guns drawn on them? The scenario being presented is also quite a bit more tense than that too in that the civilian is warning the soldier that if they advance they will be fired upon. Mind you, that's not a necessarily combat situation either. In a combat situation being openly armed civilian bystander, that just doesn't work at all.

@DKNguyen I've never been in combat. But I think that if the weapon were not in a firing position, the soldiers would not automatically draw on them. But if the local appeared to be drawing their weapon, the soldiers would respond in kind.

@Barmar I'm not sure either way. I just did watch an interview with a reporter(?) tagging along with soldiers in a truck. And the reporter asked the driver why everyone was always backing away from them and avoiding them. The driver told him that the soldier up top manning the machine gun was aiming at the civilians (unarmed) and that was why everyone was always backing away.

@Barmar I should also mention that the scenario I had in my mind was a little more dicey than your Afghanistan or WW2 fields of Western Europe scenario. I was thinking something like if German soldier encountered a civilian trying to defend their family in Stalingrad. The Soviet citizens were very much just "trying to defend their neighbourhood" there.

@DKNguyen I think there's a difference between defending your family and defending your country. If the person is acting just like they would to protect against an armed robber, they should be treated like a civilian acting in self-defense.

@DKNguyen It might not stand up in the ICC, but I like the "if it walks like a duck" test.

@Barmar occupation forces are the authorities of the occupied territory. Using weapons against them is as legal as using weapons against the police. If they want to enter your house to, for example, check that there are no soldiers or weapons hidden, they can do so. Resisting by force can be answered by force, and they would be in the clear.

@Barmar of course, self-defense could be a valid defense, but that would only apply if we are speaking about soldiers who break the rules of war and start plundering, raping and murdering civilians. It would not apply to soldiers looking for soldiers or weapons. It would not apply to soldiers trying to enter your house during active combat. You remain a civilian by acting like a civilian, and not like a Rambo...

@Barmar and for "if the [civilian's] weapon were not in a firing position, the soldiers would not automatically draw on them." seems incredibly naive. Occupying forces, including those from the USA, are known for being fast in using force against anything that could be een remotely suspicious. Famously, many journalists have lost their lives at the hands of USA soldiers, and those are the known issues because journalists cannot as easily be dismissed as "terrorists" as locals.

@Barmar You’re not approaching this from the perspective of a soldier. Imagine you’re part of an invading force in occupied territory. The area is under military law and nobody but the invading army is supposed to be armed. You know the locals don’t want you there, and you know there are partisans mixed in with the civilians and actively engaging in guerilla tactics. Are you seriously telling me that in such a situation you would go around treating people who obviously have guns as civilians?