Britain and America have a shared objective in prosecuting and convicting those responsible for crimes committed in the former Islamic State. Some of these crimes are so heinous that the Americans want to reserve the right to impose the death penalty on those found guilty. But the British government is opposed to the use of capital punishment both here and abroad. These differences of opinion have given rise to some tough talking between Britain and America over the sharing of evidence that the Americans need in order to prosecute British-born Islamists suspected of being in the notorious ISIS cell known as the Beatles. Last month the UK Supreme Court weighed into the dispute with a judgment that highlights why law should butt out of politics.

The murder of Alan Henning shocked the world. He was the taxi driver and father-of-two from Salford, who worked as an aid worker in Syria. In 2014, he was captured and murdered by ISIS terrorists who beheaded him on camera and then posted the video on the internet. The then prime minister, David Cameron, captured the nation’s mood when he described the killing as ‘absolutely appalling’ and Henning’s murderers as ‘truly repulsive’ and ‘barbaric’. Cameron vowed that ‘with others we must do everything we can to defeat this organisation. We must take action against it and we must find those responsible.’ Cameron recognised that joint national action would be needed to bring Henning’s killers to justice, and he promised to provide British assistance. Since 2014, British and American cooperation has been pivotal because those suspected of Henning’s murder are British-born and have ended up facing an American trial, which is unlikely to succeed without British evidence.

Of the four ‘Beatles’ in the ISIS cell, Mohammed Emwazi, known as Jihadi John, was killed in an airstrike in 2015 in Syria. Another, Aine Davis, is imprisoned in Turkey on terrorism charges. But the other two are thought to be El Shafee Elsheikh and Alexanda Kotey. They are currently held by the American military in Iraq. Elsheikh and Kotey are suspected to have been involved in beheading more than 27 hostages, including Britons Alan Henning and David Haines, and US citizens James Foley, Steven Sotloff and Peter Kassig. The British extremists were known for their brutality. They repeatedly beat the hostages they imprisoned in Raqqa, Syria – formerly the Islamic State’s self-declared capital – and subjected them to waterboarding and mock executions.

Britain and America made a treaty for mutual legal assistance in criminal matters in 1994, which commits both governments to assist the other with criminal proceedings. In 2015, America invoked this treaty to ask Britain to supply material gathered by UK police as part of a British investigation into the Beatles. That evidence is thought to be critical to any prosecution. In 2018, an American-backed Kurdish militia, the Syrian Democratic Forces, captured Elsheikh and Kotey. At this point, there was much hope in America that the British evidence would be handed over. But despite the enormity of the crimes being investigated – and the fact that Britain and America are allies with a shared desire to convict terrorists – the Americans had to push hard to secure the relevant evidence. The stumbling block was that two of the offences under American investigation (homicide and hostage-taking resulting in death) carry the death penalty. By custom, Britain will not provide mutual legal assistance in cases where the death penalty is a possible punishment, without a written assurance from the requesting state that a capital penalty will not be carried out. However, this is only a custom, it is not a rule of law, and it can be waived according to circumstances.

Between 2015 and 2018, the British request for a no death penalty assurance was not forthcoming. At one stage, the UK ambassador in Washington advised that if Britain persisted with seeking this assurance, then the American response was likely to be ‘close to outrage’. In part, this was because, according to the ambassador, the Americans felt ‘that we are dumping on them a problem for which we should take responsibility’. Moreover, the Americans took the view that a prosecution in a civilian court would not succeed without the British evidence. America’s alternative was to try Elsheikh and Kotey in a military court or at Guantanamo Bay, but these options were even less acceptable to the British authorities. In a presentation to a Senate panel hearing in 2018, the American attorney general, Jeff Sessions, portrayed the UK stance as one of unwillingness to try the two British-born terrorists, while also seeking to dictate how they should be tried in the US. A diplomatic rift was averted when common sense prevailed in London. The then foreign secretary, Boris Johnson, noted that the case was ‘unique and unprecedented’ and that it was ‘in the UK’s national-security interests to accede’ to the American request, even without death penalty assurances. A political problem had been resolved at a political level, with two nations agreeing that their differing views of capital punishment would not stand in the way of their common goal.

It ought to have been the end of the matter, but it wasn’t. Last month the UK Supreme Court ruled that the British government had acted unlawfully in handing over the witness statements. Furthermore, the Supreme Court didn’t beat about the bush in doing what it could to hamper further cooperation between Britain and America. Its ruling urged the government not to provide any further material and to now refuse to permit state-employed witnesses, such as police officers, to travel to America to give evidence. Absent a constitutional stand-off between the government and the judiciary, the court’s injunction will surely hold sway, unless the Americans now cave in and abandon the right to use the death penalty. This seems unlikely.

The judgment from the Supreme Court is shocking. Seven judges have waded into a political issue of the most sensitive kind. They ruled in favour of a claim brought by the mother of El Sheikh, a man suspected of what the court described as ‘monstrous’ and ‘heinous’ crimes. As one American law professor put it: ‘This is a big deal. The decision is a tremendous blow to the US government’s plan to prosecute the Beatles in an American court.’ The former British prime minister’s promise of cooperation to find those responsible for Henning’s beheading has been shattered. In an earlier era, the law would have left such political issues to be resolved by politicians. The implementation of a bilateral mutual-assistance treaty, which gives government ministers the right to decide whether to cooperate with a foreign state, is a quintessentially political issue. The Divisional Court threw out the case last year after noting that ‘the conduct of international affairs is a paradigm example of an area in which the courts recognise the institutional competency of the executive’.

But the Supreme Court saw it differently. Lady Hale expressed her own view that the death penalty is ‘immoral and unacceptable’. She used this personal political opinion as the basis for her declaration that cooperating with the American judicial process was unlawful. Hale and her judicial colleagues reached a conclusion by elevating the importance of their own distaste for capital punishment, while disregarding the importance of other political considerations, such as American judicial sovereignty, diplomacy and the broader battle against terrorism. The boundary between politics and law has been totally erased. Almost anything decided by the government can now be trumped by judges. When issues of diplomacy are resolved by judges, the law can do untold harm to Britain’s relations with other nations. And when issues of politics are resolved by judges,with judgments based on their own political opinions, the law is free to diverge from mainstream public opinion.

In this new era, there are some who believe the law’s purpose is to challenge public opinion. They should be careful what they wish for. Buried under a wealth of news about coronavirus, the Supreme Court’s judgment received little press attention. But in a democracy, people do not take kindly to judges ruling on issues that should belong to politicians who are accountable to them. Fettering the prosecution of extremists who may be responsible for the most heinous, repulsive and barbaric of crimes is a surefire way of turning the people against the law. Jon Holbrook is a barrister. Follow him on Twitter: @JonHolb