The just war tradition grew up around armies meeting on battlefields. Two developments now stretch it. One is old but long neglected: what justice requires once the fighting stops. The other is new: weapons that let a state kill without risking its own people, and weapons that may one day choose their own targets.

Justice after war

Classic writers said surprisingly little about endings. In recent decades philosophers such as Brian Orend and the historian Gary Bass have argued for a third category beside the two Latin ones. Jus post bellum concerns justice in ending wars and in what follows them. A just peace should vindicate the rights that were violated, avoid punishing a whole population and be proportionate. A peace that humiliates and impoverishes a defeated people may plant the seeds of the next war; a peace that leaves aggression unpunished may invite the next aggressor.

Occupation raises its own duties. The Hague rules of 1907 require an occupant to "take all the measures in his power to restore, and ensure, as far as possible, public order and safety, while respecting, unless absolutely prevented, the laws in force in the country." An occupying power must restore public order and safety while respecting the laws already in force.

The 2003 occupation of Iraq is often cited as a warning. Within weeks the occupation authority dissolved the Iraqi army and barred many members of the former ruling party from public jobs. Critics argue that dissolving Iraq's army and purging the ruling party fed the insurgency, by throwing hundreds of thousands of armed and angry men out of work at once. Defenders replied that the army had largely melted away already and that keeping the old party's officials would have betrayed the regime's victims. The debate shows how hard post-war justice is in practice.

Drones and targeted killing

Armed drones let operators thousands of miles away watch a target for hours and strike with a precision older aircraft could not match. Supporters argue that this can mean fewer civilian deaths and no risk to one's own soldiers. Critics raise several worries. Some strikes were aimed not at named individuals but at patterns. Signature strikes target people based on patterns of behavior, without knowing their identities, such as men of fighting age gathering at a particular place, which can turn a wedding or a funeral into a target. Secrecy makes it hard to count the dead or hold anyone to account.

A deeper worry concerns the decision to use force at all. A war that costs a nation's own citizens nothing, critics fear, will be too easy to start and too easy to continue. Critics worry that risk-free force lowers the threshold for using force at all, so that the jus ad bellum tests are skipped because the costs seem invisible.

Autonomous weapons and cyber operations

The next step would remove the human from the final decision. Autonomous weapons, once activated, would select and attack targets without further human input. Supporters argue that machines feel no fear, panic or desire for revenge, and might one day apply distinction more reliably than exhausted soldiers. Opponents reply that a machine cannot recognize surrender or weigh proportionality in a new situation, and that being killed by an algorithm offends human dignity. The philosopher Robert Sparrow pressed another objection. The responsibility gap is the worry that no one may be properly accountable when an autonomous weapon does wrong: not the programmer, who could not foresee the case, not the commander, who did not choose the target, and not the machine. Talks at the United Nations on regulating such weapons have run for over a decade without a treaty.

Cyber operations raise their own questions. When does disabling a power grid by code count as an armed attack? Is a hospital's computer system protected like a hospital? A group of legal experts convened in Estonia produced an influential answer. The Tallinn Manual sets out experts' views on how existing international law applies to cyber operations.

Worked example: a strike in Kabul

On 29 August 2021, during the final days of the American withdrawal from Afghanistan, three days after a suicide bombing at Kabul airport killed about 170 Afghans and 13 American troops, a US drone fired a missile at a white car pulling into a family courtyard. Analysts had tracked the car for hours and believed its driver was preparing another attack. The driver was Zemari Ahmadi, a longtime worker for an American aid group. Containers he had loaded turned out to hold water. Ten people died, seven of them children. Weeks later the commander of US Central Command called the strike a tragic mistake, and the Pentagon later decided that no one would be disciplined.

The case gathers the course's threads. Distinction failed because analysts read innocent acts as hostile ones under pressure. Double intention asks whether more could have been done to confirm the target before firing at a home. The drone removed risk to American personnel, but not to the people below. And the absence of accountability tests the claim, central to the tradition, that the rules of war bind those who make them.