Even war has rules. Within two days of Russia invading Ukraine, the International Criminal Court was investigating reports of war crimes there, including the bombing of schools and hospitals.

The United States has already formally denounced Russian President Vladimir Putin as a war criminal over the invasion, and UK Prime Minister Boris Johnson and Australia’s Foreign Minister Marise Payne say he must be held to account for war crimes against Ukrainian civilians. Russia has been accused of using weapons banned in other countries such as cluster bombs, and there are fears the invaders may yet resort to chemical weapons or increase attacks on nuclear plants, as they step up bombardment of cities in the face of fierce Ukrainian resistance.

So what might a war crimes prosecution involve – and could Putin find himself on trial at the Hague?

What are war crimes?

The first war criminal on record was tried (and executed) long before the Geneva conventions that govern warfare today were drafted. (His name was Peter von Hagenbach, a Burgundian knight convicted in 1474 by an tribunal of the Holy Roman Empire for atrocities during the occupation of Breisach.)

Soldiers and nations at war operate under the Geneva conventions, a code first established in 1864 and extended since the world wars, as well as other international treaties. The laws of war are not designed to end a war, explains UWA Law School’s Dr Melanie O’Brien, but to protect those not taking part in the fighting such as civilians, doctors, wounded troops or prisoners of war.

“People will kill other people during conflict,” she says. “What’s not allowed is murder.” It is illegal to deliberately blow up a hospital, for example, use civilians as shields or kill someone who is “hors de combat” (out of combat). “The target must be a military objective,” O’Brien says.

“It’s quite striking how rampantly Russia is violating” the treaties they are party to, she notes. As well as bombing schools and hospitals and kidnapping mayors, Russia has attacked high-risk nuclear power plants. It has used indiscriminate weapons on cities – “dumb bombs” without precision targeting and cluster bombs that contain many munitions. It’s as if Russia “has a checklist,” O’Brien says, “and they’re going through, ticking off the laws of war and violating them”.

Who prosecutes war crimes?

The ICC was set up 20 years ago in the Hague with the authority to prosecute war crimes, genocide, and wider crimes against humanity (such as the murder, torture or rape of civilians). Its chief prosecutor, Karim Khan, is now in Ukraine gathering evidence and told CNN that “the law is clear on this, it is a crime to intentionally target civilians [and] civilian objects”. But he added that the burden of proof remains high – beyond reasonable doubt – and the process is yet to play out.

Deliberately targeting civilians is different from killing them in the “fog of war”, in the confusion of battle, when a civilian reaching for their mobile phone could be mistakenly thought to be drawing a gun.

In Ukraine, Khan said the court would also consider “whether civilian objects [were] being used to launch attacks that made them legitimate targets”. “But even then, it’s no licence to use cluster bombs or use disproportionate attacks in concentrated civilian areas.”

Most war crimes are prosecuted after the fact, says international law expert at the ANU Professor Donald Rothwell. A live investigation, such as Khan is conducting on the ground, is more difficult. “The court will have to proceed with caution,” Rothwell says. “Civilians, for example, cannot be combatants [until] they pick up a rifle and start fighting, then they may lose their immunity.”

But the court could potentially intervene in war crimes as they are unfolding. “Say if a Russian military commander falls into the hands of the Ukrainians, and they pass them onto the ICC,” Rothwell says. Still, what difference successful prosecutions would make on the war’s progress is unclear.

Who can be prosecuted?

Anyone can be held accountable – from the lowest foot soldier to the country’s leader. The ICC tends to focus on leaders and commanders because resources are limited, O’Brien says. In the case of war crimes allegedly committed by Australian soldiers in Afghanistan, decisions under the spotlight relate to individual soldiers, says Rothwell. But in Ukraine, the focus is likely to be on Russian generals and Vladimir Putin.

It can be hard to prove direct responsibility higher up the chain of command but leaders have been convicted before, including Liberia’s Charles Taylor for sponsoring atrocities in neighbouring Sierra Leone. Former Yugoslav leader Slobodan Milosevic was also put on trial by a UN tribunal but died before a verdict. (His Bosnian Serb ally Radovan Karadzic and the Bosnian Serb military leader Ratko Mladic are both serving life sentences.)

Will Putin ever be tried for war crimes?

If the court finds sufficient evidence to try Russia’s President, it could issue an arrest warrant. The court has no police powers and countries who do not recognise it such as China, Russia and the US would have no obligation to hand over Putin. The court does not try people in absentia except in exceptional circumstances, says Rothwell. “But it’s a long game. If there’s a change of regime in Russia, if Putin is forced out or steps down, who knows what pressure a new leader may come under to hand him over.”

O’Brien says international examples suggest that 69-year-old Putin’s position may not be so secure forever. Former Sudan president Omar al-Bashir was wanted for war crimes, crimes against humanity and genocide by the ICC in 2008 but remained in power until 2019, when a coup brought him down. After being jailed for corruption in Sudan, he is expected to be handed over to the ICC for trial.

Some countries have their own laws for prosecuting war crimes. Germany and Ukraine, for example, are already investigating.

Meanwhile, a growing chorus of experts, led by former UK prime minister Gordon Brown, is calling for a separate tribunal, modelled on the Nuremberg trials, to go after Putin for the crime of aggression – invading a sovereign nation – which the ICC cannot prosecute itself without special leave from the UN Security Council (where Russia holds veto power). It would take enormous political will to launch such a tribunal among like-minded countries, Rothwell says. “But if the US put diplomatic support behind this, it could build momentum.”

On the ground in Ukraine for the ICC, meanwhile, Khan is hopeful that international justice will prevail. “We’ve seen time and time again, from Nuremberg onwards, the truth will out, and that gives me confidence,” he said.

“This is an opportunity to mobilise the law and send the law into battle, to protect and to deter, and to insist on accountability. Because if we don’t do this, we’re going to keep on having the sweet nothing’s of never again, of wringing our hands. And knowing what is coming tomorrow because we saw it yesterday.”