In early February, when Vladimir Putin’s troops were on the Ukrainian border and much of the world thought he was bluffing, the Russian military’s guidance on mass graves was changed. Bodies should be covered with chemicals, diagrams showed, and then rolled over by a bulldozer to flatten the ground. The advice seemed so grotesque as to be a decoy: surely a brutal invasion would not be so clearly signalled?

The story of the mass grave found in Bucha shocked the world because it represents how fast things have deteriorated and that we are now seeing the kind of barbarism Europe thought it had left behind. The pictures of dead children and the corpses with their hands and feet bound were proof that nothing has changed, and a reminder of our impotence: all of Ukraine’s allies have ruled out direct conflict with Russia.

Right from the start of the conflict, Volodymyr Zelensky tried to persuade his allies that Putin should be recognised not just as an aggressor but as a war criminal. This week he repeated the message. ‘Civilians were crushed by tanks while sitting in their cars in the middle of the road,’ he told the United Nations. ‘Women were raped and killed in front of their children.’ President Biden has said that war crimes proceedings should start – but to what end? It sounds, as one British minister puts it, ‘a bit like virtue signalling. What are the chances of Putin ending up in the dock? Or any Russian government sending Russian soldiers to stand trial?’

This is, nonetheless, a new front in Britain’s war effort. Dominic Raab, the Deputy Prime Minister, has offered £1 million to the International Criminal Court in The Hague to begin the process of prosecution. Suella Braverman, the attorney-general, is working with Iryna Venediktova, Ukraine’s prosecutor–general, to identify and bring charges against Russians accused of atrocities in Mariupol and elsewhere under Ukraine’s own court system. ‘This work may take years, perhaps decades to complete,’ says one UK lawyer assigned to the mission. ‘But done properly, it can make a difference now.’

Even if no more mass graves are uncovered in Ukraine, the massacre at Bucha would be the worst in Europe since Srebrenica in 1995, when 8,000 Bosnian men and boys were killed by Serbs. This happened under the watch of the United Nations, who had declared the town a safe area. Some 6,800 victims were later identified through DNA analysis of body parts, and their deaths were prosecuted as murder.

At the time, the prosecution appeared to be a fool’s errand: there seemed no realistic prospect of prosecuting individual camp guards let alone Radovan Karadzic, who had ordered the siege of Sarajevo and overseen Srebrenica. But as the years passed, the case for the prosecution was built, and in 2008 Karadzic was arrested. Slobodan Milosevic died of a heart attack while awaiting trial but Karadzic is now serving a life sentence in the Isle of Wight prison. Charles Taylor, the former president of Liberia, who was convicted of war crimes committed in Sierra Leone, is in Frankland prison in Durham.

For advocates of the International Criminal Court system, this is proof that the system can work – even if at first the odds seem hopeless and the process takes years. ‘At the very least, it’s a travel ban,’ says one senior UK official. ‘If we name the soldiers and generals behind these atrocities, they risk arrest if they ever leave Russia. We want Russian conscripts to know that this is an illegal war, and rather than be feted, they might face prosecution. Generals should ask themselves if they can bet that Putin will be in power in 15 years’ time. If not, they could be vulnerable. The idea is to sow doubt.’

In his presentation to the United Nations this week, Zelensky released a chilling montage showing the bodies of dead civilians in Motyzhn, Irpin, Mariupol and Bucha. This footage can’t necessarily be used in court. Social media videos are easy to misrepresent and manipulate and proof of death is not proof of a war crime. In a conflict where all manner of people take up arms (officials in Kyiv estimate a million Ukrainians are fighting Russians) the line between soldier and civilian can be blurred.

Hence the need for forensic evidence – and for British help. The UK Ministry of Justice is offering to assist in Ukraine’s investigation work, dispatching lawyers to interview witnesses and gather evidence that will stand up in court. SO15, a division of the Metropolitan Police, is already taking evidence from Ukrainians and processing it in a way that would withstand legal challenge. This is quite a job: of the 3.6 million videos submitted as evidence for Syrian war crimes, just 600,000 have been analysed and barely 8,000 verified as being fit for court. The assessment work is monumental, and well beyond the capability of a Ukrainian government that is still fighting for survival.

Sir Howard Morrison, who as a judge convicted Karadzic, has now been seconded by the UK government to help Ms Venediktova build her case. She is planning to name dozens of Russian soldiers within the next few weeks and believes she already has the proof she needs for a trial in Kyiv. The chances of getting hold of defendants may be slight, but the intention is to make it clear that these war crimes won’t be forgotten. This is an era of mass digital evidence. Ukrainians can document every atrocity, and every Russian order can be intercepted and exposed. As the Kremlin found out after Bucha, denials are meaningless when satellite images show bodies lying strewn in the streets during the Russian occupation.

British and American intelligence officers were not among those who thought Putin was bluffing back in February. They knew what was about to happen and they know now which Russian officers are issuing which orders.

The wheels of justice at The Hague may turn very slowly, but with parallel investigations in Kyiv (and perhaps others in the Baltic states) the body of digital evidence will grow fast. Already, documentary evidence for hundreds of cases is being published online.

All this has a wider political purpose. It sends a message not just to Russia but to the world. If the Ukraine invasion is seen as a war crime as well as a military conflict, this may persuade countries like India and South Africa to abandon their neutrality. It could persuade Putin’s critics in Moscow that this ‘special military operation’ is of a different order to the operations in Georgia and Crimea.

For Zelensky, the war crimes evidence serves a different purpose. It is proof for him that his allies need to go further. They should tighten sanctions and send offensive weapons capable of repelling Russians.

And what of Putin himself? Here, every-one is more cautious. Even now, the US has not accused him personally of any charges. ‘You have to leave him enough rope to climb down,’ says one No. 10 source. ‘If you say Putin is a war criminal, there is no way out for him.’ A war criminal could not attend summits, as he would be arrested if he steps outside his country. ‘Your president stands accused of committing war crimes,’ said Boris Johnson in a video message to Russians. He was careful not to make the accusation himself.

It’s hard to imagine that Putin will ever stand trial. An arrest warrant could be issued by the International Criminal Court at The Hague, but its authority is not recognised by Russia or the United States. The United Nations runs the International Court of Justice, also in The Hague, which can rule against countries, but any ruling has to be enforced by the UN Security Council, of which Russia is a veto-wielding member.

Putin’s calculation will be that, just as after Syria and Grozny, talk of his war crimes will be forgotten. The challenge for Ukraine and its allies will be to assemble enough evidence quickly enough to make a diplomatic difference. The odds are slim – Ukraine needs all the help it can get.