The final unravelling of the Islamic State’s evil caliphate exerts a horrible fascination. The jihadis committed many appalling crimes in Syria and Iraq – exploiting the chaos caused by the Syrian civil war – and were responsible, directly or indirectly, for murderous attacks in Britain and several other European countries.

Most people expect a reckoning. It is only right that Isis fighters who have been captured alive, and those who gave them aid and succour, should face justice as soon as possible.

The actions of many other individuals and groups suspected of involvement in war crimes and crimes against humanity, such as the killing of non-combatants, should also face rigorous scrutiny. In the interest of impartiality, the list must include rebel Sunni Arab and Kurdish opposition forces.

Nor can the role of western and Gulf state governments be air-brushed away. The Ministry of Defence’s claim last week that RAF airstrikes killed or wounded 4,315 enemy fighters in Iraq and Syria since 2014, but killed only one civilian, beggars belief.

Yet the quest for justice for all must not obscure the fact that the main responsibility for eight years of mass killing lies squarely on the shoulders of Syria’s dictator, Bashar al-Assad, his regime cronies, and their Russian, Iranian and Lebanese Hezbollah allies. In his bid to cling to power, Assad has presided over the deaths of up to 500,000 people, the displacement of roughly half of Syria’s prewar 23 million population, the destruction of major cities and towns, and a vast, destabilising humanitarian crisis.

Assad and his main foreign backer, Russian president Vladimir Putin, are responsible for the merciless, systemic and endlessly repeated aerial bombing of schools, clinics, hospitals and community centres designed to terrorise and cow local populations. Captured opposition fighters and sympathisers have been tortured and murdered in their thousands. Assad, supported by Moscow, has used chemical weapons more than 100 times in flagrant breach of legal commitments made in 2013.

There is no shortage of evidence to back these charges. The “independent, impartial and international mechanism” for investigating serious crimes in Syria, created by the UN general assembly in 2016, has documented thousands of crimes. The so-called Caesar archive – the regime’s own photographs of the bodies of 6,700 people who died in custody – was smuggled out by a defector. Then there is the eye-witness testimony of exiled Syrian civilians and of UN and NGO workers and human rights activists.

The problem is not proving the guilt of Assad and his associates, but bringing them to trial. Attempts in the UN security council to empower the International Criminal Court (ICC) to investigate Syrian war crimes have been blocked by Russia. Assad sits pat in Damascus and denies all allegations as “fake news”. He doubtless takes comfort from the ICC’s eight-year-long failure to try Omar al-Bashir, Sudan’s president, for alleged genocide in Darfur.

But there are glimmers of hope. National courts in Europe are under pressure from activists to employ rarely used powers of universal jurisdiction to pursue crimes against international law, such as those committed in Syria, even where there is no direct domestic connection. Courts in France, Germany and Sweden are currently investigating several Syrian individuals, including regime intelligence officers, suspected of serious crimes.

Last week a group of Syrian refugees, advised by British barristers and solicitors, launched a legal bid that would enable the ICC to get involved. Following a precedent set last year in Myanmar that extends ICC jurisdiction to cover forcible population transfers, the group wants the chief prosecutor to use their dossier of evidence on regime massacres and torture to open a case against Assad.

It is a valiant effort deserving of success. Yet it is a shocking indictment of the UN-led international system, and the inaction of individual governments, including Britain’s, that this may yet prove the best hope of bringing Assad to book.