John Demjanjuk, currently being tried for war crimes in Munich, is accused of having served as a guard at the Sobibor death camp, where at least 167,000 Jews from Poland, the Netherlands and France were murdered in 1942-43. The evidence against him was strong enough to secure his denaturalisation and deportation from the United States. German prosecutors are convinced they can prove he was at the camp and complicit in the killing of at least 29,000 Jews. But even if he is found guilty, will the trial deliver anything approximating to justice?

For connoisseurs of German justice, in particular, the way the case is evolving is bittersweet to say the least. During the 1950s, 1960s, and 1970s it required the judicial equivalence of a siege engine to bring to trial men suspected of committing crimes under the Nazis. Not only was the process protracted, those unfortunate enough to be convicted of working as guards at concentration and death camps, or serving in mobile killing units, often received light sentences.

Because they were taking orders and were calmly carrying out a state policy of genocide, it was hard to convict them of homicidal acts fuelled by hate or cruelty. Sentences of seven to 15 years for complicity in mass murder were common, and were often reduced to take into account periods of detention as prisoners of war in the USSR after 1945, or time in jail during the judicial process. Many Germans and foreign observers regarded West German justice as a joke when it came to reckoning with the Nazi past.

So this new earnestness, while welcome, is somewhat surprising. What has changed? Firstly, Demjanjuk is not a German. For once a German court has the opportunity to lay out the crimes of the collaborators, to expose the degree to which Nazi mass murder relied on non-Germans. Secondly, the federal German central office for the prosecution of Nazi crimes wants to show its mettle and continuing relevance. The German investigators and prosecutors who drove this case forward seized on Demjanjuk to demonstrate that there was still unfinished business from Nazi times, that they still have jobs to do. Finally, Demjanjuk is an old, old man. There was simply no room for prevarication or legal hand- wringing.

Yet herein lies the dilemma haunting the trial. If it lasts beyond April 2010 Demjanjuk will be 90 years old. Even if the court operates at hyper speed, the process is unlikely to end before May next year. The prosecution has lined up the few survivors of the camp well enough to testify, as well as about 30 relatives of those who crossed its lethal perimeter. Hearing them all will take time. In order for the trial to fulfil its educative function it cannot be too rushed.

However, what court that claimed an ounce of humanity would send an ailing nonagenarian into a cell – or more likely a prison hospital – for the last of his few years on earth?

I used to be a partisan for war crimes trials, and argued that time never absolved the guilty of their crime, least of all the crime of genocide. But if Demjanjuk is found guilty, which is the almost inevitable outcome, I cannot see how justice will be done.

The story of Sobibor will have been told in awful detail. The survivors and their families will have had their day in court. The world, which thinks itself saturated with information about the fate of the Jews during the Second World War, will have learned more.

But how can the gravity of the offence be signified in a sentence on a sick, 89-year-old man? What punishment can match the crime that will not also dismay those with a spirit of compassion? Alternatively, if the court shows mercy (of the kind Demjanjuk never spared for his victims) and follows the pattern of earlier trials, he will receive such a mild sentence as to make the whole business seem ridiculous. This may be the last trial of a Nazi collaborator stained with mass murder, but if it ends with either a harsh sentence, or a sign of humanity, it may prove to have been a trial too far.

David Cesarani is research professor in history at Royal Holloway, University of London and author of 'Eichmann: His Life and Crimes'