Our international obligations, Andrew Wallis says, demand that UK residents accused of Rwandan atrocities must be extradited or else tried in British courts, with all the challenges that entails.

Westminster magistrates court is a world away from Rwanda or the horrors of its genocide, which began 20 years ago this week. But it is in this incongruous setting that the international search for justice against those responsible for the massacre of 1 million people in 100 days now moves.

Five men are fighting extradition to Rwanda, where they would face charges of organising and taking part in the slaughter. The result of the case, scheduled to last until June, will have a major impact not just on the hopes of genocide survivors but also on the responsibility of the UK to meet its international pledge that those accused of the most terrible crimes will not escape justice.

We know from the UK and other European countries that the case will follow a familiar pattern, one of politics put before justice. Indeed, four of the five men previously escaped extradition in 2008 by arguing that they would not face a fair trial back in Rwanda.

In this case, the defence strategy will be to put the Rwandan government in the dock. Rwandan political dissidents now in exile, francophone academics and former French military officers are called as expert witnesses. They invariably admit they don't know the accused or whether they are innocent or guilty. Instead they take the stand to decry the current Rwandan government and the president, Paul Kagame. The genocide and alleged crimes of the defendant are secondary. It is a well worked formula seen in genocide trials during the past two decades, from Arusha to Paris, Oslo to Quebec.

This time, however, they may find it more difficult to convince the UK court. Since 2008, the Rwandan justice system, helped by funding and expert advice from Europe, has been totally remodelled. It has proved such a success that the UN, Canada, Norway, Denmark and the Netherlands have all recently agreed to return suspects for trial. The European Court of Human Rights has also judged the system to be free and fair.

It is not possible, of course, to predict what the ruling may be this time, and a decision to oppose extradition should not be the end of the matter. The UK ratified the 1948 Convention on the Prevention and Punishment of the Crime of Genocide. This obligates the UK to ensure those accused of genocide stand trial, if necessary, in its national courts, no matter how costly and time-consuming this proves. This is the route being taken in France and Germany.

For this reason, the case this week thrusts into the spotlight the challenge for the UK and other western justice systems of what to do when all those accused of mass murder abroad flee to its shores and claim political asylum or just 'disappear' into diaspora communities. Recent civil wars and human rights abuses abroad have only led to an increase in those fleeing to the UK and other European countries. Where extradition is denied, trying them in domestic courts also poses great difficulties.

We have seen in extradition cases in the UK and the Rwanda genocide trials in Paris and Frankfurt how difficult it is to judge alleged crimes committed thousands of miles away in conflicts and contexts that are poorly understood. Solicitors struggle to pronounce Rwandan names, locate places, or make sense of distances. Language difficulties mean that witnesses, whether appearing in person or by video link, are barely understood. Even after legal fact-finding visits, cultural and historical knowledge is lacking.

If the delays in bringing perpetrators to justice has been difficult for survivors of the genocide to understand, so too is the sight of high-profile human rights organisations campaigning on behalf of those accused of these terrible crimes. Justice is pushed aside in favour of the chance to politically attack the Rwandan government. The survivors, yet again, are forgotten.

We cannot allow this injustice to continue. The unhealed wounds from 1994 genocide will only begin to close when the west becomes serious about justice. It cannot continue to operate a 'two-tier' justice system that means horrendous crimes committed outside its own geopolitical sphere are not punished or taken seriously while it moves every stone possible to bring alleged offenders against its own citizens before the courts.

The events at Westminster magistrates court are likely to be replayed many times in the next few years. The UK must play a full part in bringing timely justice, whether through extradition or internal trials, in such horrific cases as these, which see humanity itself on trial.