Is a machine that automatically shoots back legal?

Question

A major difficulty in modern warfare is that militants often shoot rockets from within a civilian area, then quickly go into underground tunnels. It is very difficult for the defenders to shoot back at the attackers; it is possible to bomb the area the rocket was shot from, but this will probably not harm the attackers - it will only harm uninvolved citizens, which arguably violates the laws of war.

Now, suppose country A has a super-advanced machine, that can detect a rocket shot from country B with 100% precision, and automatically shoot back to the place the rocket was shot from. This will still probably not harm the attackers - it will only harm uninvolved citizens of country B. The difference is that, now, the harm to citizens of country B is done automatically, triggered only by the militants of country B, without any intervention of country A. Does country A violates the laws of war by installing such a machine?

Second question: suppose country A has an even more advanced machine that, once it detects a rocket shot from country B, deflects it back to country B (using a super-strong force-field), where it falls and harms uninvolved citizens of country B. Does country A violates the laws of war by installing this machine?

EDIT: in both cases, I assume that everyone in country B knows about this automatic system. So when militants in country B shoot at country A, they knowingly trigger the counter-shoot at their own citizens. My argument is that, although the international law is violated, the violators are the militants from country B who trigger the shoot - not country A who placed the system in advance. Is this reasoning correct?

Answer 1

Who is the primary target of the machine?

• If civilians, as the act of revenge, the machine is illegal by the laws of war. Intentional targeting of civilians is not permitted.
• If the machine is aiming at the enemy combatants that may be still in place, the machine is legal and possible to use, even if it kills some civilians as well.
• If the machine aims to combatants but works so badly that mostly kills civilians anyway, it may be illegal because of doing disproportional damage to civilians.

I think it does not matter is the machine automatic or not, or by whom it is triggered (bobby trap is also an automatic device technically triggered by the victim). Who deploys or operates the machine remains responsible for its actions.

Answer 2

Such devices would probably be subsumed under the general rubric of Lethal Autonomous Weapons Systems, which can be legal. A negative view of such systems from António Guterres says that they "are politically unacceptable and morally repugnant and has called for their prohibition under international law", which of course says that they are currently legal under international law. The US has taken a position, addressing three legal desiderata:

(a) Distinction. In that, “Combatants may make military objectives the object of attack, but may not direct attacks against civilians, civilian objects, or other protected persons and objects.”

(b) Proportionality. In that, “Combatants must refrain from attacks in which the expected loss of life or injury to civilians, and damage to civilian objects incidental to the attack, would be excessive in relation to the concrete and direct military advantage expected to be gained.”

(c) Precautions. In that, “Combatants must take feasible precautions in planning and conducting attacks to reduce the risk of harm to civilians and other persons and objects protected from being made the object of attack.”

In principle, such a system could be legal, as long as the device distinguishes combatants from civilians and provides a proportional response. There is no requirement that civilians be 100% protected.

Answer 3

Spring guns are mines

A spring gun cannot distinguish between a combatant or any non-combattant. Because it can and will shoot civilians, that can violate the geneva convention:

Article 27 of the Fourth Geneva Convention:

Protected persons are entitled, in all circumstances, to respect for their persons, their honour, their family rights, their religious convictions and practices, and their manners and customs. They shall at all times be humanely treated, and shall be protected especially against all acts of violence or threats thereof and against insults and public curiosity.

That reasoning would also go for mines, so why are mines not treated as illegal? Well... Anti-personnel mines are banned by separate treaties (especially the Ottawa Treaty). Still, the rules and customs of war allow mines to some degree. However, the CCW Amended Protocol II demands that minefields have to be clearly marked with small red triangles indicating the presence of anti-personnal mines, to reduce civilian casualties.

It would be reasonable to treat any automated gun to be an anti-personnel landmine and require the very same principles as for mines. In other words: countries signing the Ottowa Treaty are banned from using them, all others have to mark the areas where they are used. Otherwise, their use could be a war crime.

Counter Battery Fire is different

Let's make an example: Ruritania is at war with Evilstan. The 1st Chasseurs of Ruritania get shelled by the 2nd Artillery Brigade of Evilstan. The 2nd Artillery Brigade has its base on the marketplace in front of a hospital. That spot is very problematic:

The marketplace, by being a military position, would be a legitimate military target. The hospital however is a prohibited target. Targeting the marketplace would be legal, but doing so for the hospital is not. Shelling either could hit the other...

The legality of the counter-battery fire on the 2nd Artillery Brigade depends on the principles of proportionality and how discriminatory it is: Indiscriminate counter-battery fire that is either aiming or not caring for the hospital is clearly a war crime. But well-aimed counter-battery fire that does not blow up surrounding houses more than absolutely needed would be proportional and limit the civilian casualties.

That is the basis to look at when identifying automated counter-battery fire. With some work, it could be made following the rules of war, by drafting exclusion zones where the automated guns would not shoot, such as the hospital.