DETMOLD (GERMANY) (AFP) –  A German court is due to rule Friday in the mass murder case of a former Auschwitz SS guard, more than 70 years after the end of World War II.

Reinhold Hanning, 94, is accused of complicity in more than 100,000 murders at the death camp in Nazi-occupied Poland.

Here are key facts on what post-war Germany has, and has not, done to bring to justice former Nazis for Holocaust crimes — and why it is still pursuing the last cases seven decades on.

How many Nazi criminals have been convicted?

War crimes trials by the victorious western Allies, and later by tribunals in West and then reunified Germany, have convicted around 6,650 people since 1945 — only a tiny fraction of those who wore Nazi or SS uniforms — says historian Andreas Sander.

Only seven percent of these were for cases of “mass murder”, including the genocide of the Jews and other minorities.

And only nine percent of the sentences ran beyond five years in prison, including 166 life sentences. Most convictions were over denunciations or persecutions during the Nazi era.

Courts in communist East Germany were far more active, handing down 12,890 convictions between 1945 and 1989 and, on average, imposing heavier penalties, says Dutch jurist Christiaan Rueter.

However, those figures may reflect not just a desire for justice but also the realities of the Cold War, when the regime also used alleged fascist links to jail its critics and enemies in political trials, says Daniel Bonnard of Marburg University.

Why have so few gone to jail for war crimes?

In the early years after Nazi Germany’s defeat, there was a general reluctance to pursue former Nazis, many of whom remained in key administrative and judicial positions.

Collectively punishing all former Nazis would have meant putting half the country in the dock.

Political choices made in 1949, during the founding of the young democracy under western tutelage, contributed to the collective amnesia, Bonnard said.

Germans were focused on rebuilding a country in ruins, and many remained in denial about past crimes, dismissing the 1945-49 Nuremberg trials as “victor’s justice”.

The young country did not write into its legal code the notion of “crimes against humanity” which would have covered the collective dimension of Holocaust.

This only happened more than half a century after WWII, in 2002, but was not applied retroactively.

What consequences did these decisions have?

Because of the absence of this broad legal principle, “judges were stuck with common criminal law” for decades, says Andrej Umansky, penal law expert at Cologne University.

Judges were broadly sympathetic to defendants’ arguments that they had to obey orders under a dictatorship, says Umansky.

The letter of the law only prescribed life imprisonment for aggravated murder, and courts often hesitated to impose such heavy sentences on elderly defendants who were often well placed in society.

For decades, there was a staggering burden of proof, including evidence that a defendant participated directly in a crime, acted on their own initiative or with unusual cruelty.

Most suspects were regarded as “accomplices” and received lighter sentences, says Bonnard.

Why did it take so many decades to change things?

The hunt for Nazi war criminals received an initial boost in 1958, when Germany set up the Office for Investigation of Nazi Crimes in Ludwigsburg, and in 1964 when it expanded its remit.

But the process hit a stumbling block in 1968, when Eduard Dreher, a jurist with a Nazi past, quietly and subtly tweaked the law.

The change imposed a statute of limitations for complicity in murder committed with the motive of racial hatred. That change, hidden in complex legal documents, went largely unnoticed by the public but put on ice some key proceedings, including a case against several top Nazis who had been in charge of sending Jews to the gas chambers.

Why the recent spate of ‘twilight justice’?

Dreher’s actions only came to the notice of the wider public in 2012, after the lawyer Ferdinand Von Schirach wrote about it in the novel “the Collini Case”, causing a media storm.

That same year, the Justice Ministry ordered a commission of inquiry to examine its past.

Meanwhile, the case of Ukrainian-American John Demjanjuk, a former death camp guard who had long lived as an auto plant worker in the US state of Ohio — set a new legal precedent.

In 2011 a German court convicted him not for crimes he could be linked to personally, but on the basis that he had served at the Sobibor camp in occupied Poland.

The notion that everyone who served as a cog in the vast Nazi killing machine is culpable also led to the 2015 conviction of Oskar Groening, dubbed the “Bookkeeper of Auschwitz”.

Many have decried these sentences as too little, too late, as the last surviving defendants are all aged in their 90s.

A challenge to the Groening verdict is pending before Germany’s highest court on whether the element of individual action can be entirely dropped in war crimes trials.

The Constitutional Court is due to issue a landmark ruling on the question at the end of the year.