It had to be organized quickly. Just three weeks before the opening of the "Great German Art Exhibition" in Munich in 1937, which had been declared a top priority by Adolf Hitler, another exhibition was also to be prepared to demonstrate the contrast with the new national art.

The defamatory show was to present so-called "degenerate art." That included works that didn't conform to the Nazis' views about art: Expressionism, Surrealism, Dadaism, Cubism, New Objectivity and Fauvism - basically all modern art was to be pilloried and ridiculed.

Artworks were collected throughout Germany. A confiscation commission ordered by Hitler was launched on June 30, 1937, to collect the "appropriate" exhibits from different museums.

"Purging art was not a new idea. Certain lists had already been prepared by museums," explains Meike Hoffmann, project coordinator for the research center "Degenerate Art" at Berlin's Freie Universität. "Museums had already been brought into line; museums directors who refused to comply were replaced."

Nevertheless, the request took museums - and especially the smaller ones - by surprise. After all, the "Degenerate Art" exhibition was to open on July 19, 1937 - just a day after the "Great German Art Exhibition" started.

Expressionist Ernst Ludwig Kirchner. "Dunes and Sea" (1913) is now on show at the Kunstmuseum Bern

Some 650 modern paintings, drawings and sculptures were hastily confiscated from 32 museums, including works by important masters such as Wassily Kandinsky, Emil Nolde, Lyonel Feininger, Ernst Barlach or Ernst Ludwig Kirchner.

"There was a certain amount of leeway. For example, no one checked if the initial lists were complete," says Hoffmann. "The works had to be surrendered no matter who owned them. Some local politicians deliberately looked the other way, while others would vehemently require museum directors to hand over a specific painting."

The confiscations therefore took place arbitrarily and a certain degree of insecurity ruled, which is why some museum directors obediently handed over the works even before they were called upon to do so, explained Hoffman.

A turning point in Nazi cultural policy

Few of them, however, realized that the works were not just loaned for a short term for an exhibition in Munich. Along the way, they were contributing to the process of fundamentally "purging" the German art world.

"Many museum directors saw the confiscation of the works as a loan, and obtained insurance coverage for them," says Meike Hoffmann, who has been researching the period since 2006 at the Freies Universität Berlin.

The paintings, sculptures and drawings would never be returned. On the contrary, the Munich touring exhibition "Degenerate Art" marked a turning point in the Nazis' cultural policy.

Read more: Why Hitler stole paintings - and hid in one

By August 1937, modern art had already been thoroughly plundered in Germany. Over 20,000 works from about 1,400 artists were later seized during a second, more comprehensive round of confiscations. They were stored in a Berlin depot, burned or auctioned abroad.

Hitler and Propaganda Minister Goebbels visiting the Munich exhibition "Degenerate Art" in 1937

Yet some of those works were on show in Munich. To make sure visitors understood the intentions of the exhibition correctly, the walls were smeared with slogans and snide comments. "We see outbursts of madness, of insolence, botched work and degeneracy all around us," the exhibition's director, Adolf Ziegler, is said to have declared at the time. He was a painter himself and, as the president of the Reich Chamber for the Visual Arts, he personally oversaw the purge of undesirable artworks.

The exhibition, which was shown in 12 other cities afterwards, became a crowd puller. Over two million people went to see it. To this day, it remains the most-visited modern art show ever.

Many artworks lost forever

Many visitors presumably shared the ideology of the exhibition. Contemporary witnesses remember how some visitors would spit at the images. "Others, however, certainly went to the exhibition to see these works one last time," said Ulrich Wilmes, head curator of the Haus der Kunst in Munich.

His museum has to deal every day with the legacy of the "Great German Art Exhibition," as it was specifically built in 1937 to house it. To underline its 80th anniversary, the Haus der Kunst is presenting an exhibition on its own history and presenting a reworked version of the "Degenerate Art" show, with historical photos, film archives and exhibits on the construction of the building.

The Museum Kunstpalast Dusseldorf was particularly affected by the purges

The Nazis' destructive cultural policy is also currently remembered in Dusseldorf's Kunstpalast. On July 13, an exhibition opened that reconstructs art treasures that disappeared from its collection. Very few works have been saved: After Berlin and Essen, Dusseldorf was particularly affected by the purges. Over 1,000 paintings, sculptures and drawings were confiscated.

"We are showing an exhibition about a collection that no longer exists," said Kathin DuBois from the museum Kunstpalast. It was downright eradicated, she added.

Through a loan from Sydney, the large-format painting "Three Bathers" by Ernst Ludwig Kirchner can be shown in Dusseldorf for the first time since the seizure. The rest of the exhibition features the museum's own sparse inventory.

"After 1945, not much could be recovered. Some works are still considered missing, such as the painting 'The Beautiful Gardener' by Max Ernst, which was on show at the 'Degenerate Art' exhibition. Many were destroyed, especially the paintings from local and then still unknown painters," said DuBois.

Read more: 'Suspicious' art from Nazi dealer to go on show in Germany

Impact of art purges still felt to this day

Meike Hoffmann also deplores this huge loss: "Young artists who were only at the beginning of their careers have been completely forgotten, unlike the painters who were already renowned at the time."

Basically, the German art world has not only lost a number of its most valuable paintings through the Nazis' purges, but the entire development of the country's arts is still affected by them, according to Hoffmann.

"The Nazis fortunately did not manage to completely eliminate modern art; there was an intensive reparation process after 1945," she says. But that also created new problems, as the focus was on an already outdated understanding of modern art, the expert adds.

It would also be difficult to clearly separate the art created under the Nazis in black-and-white categories - as "degenerate" or promoting the party's ideology. "Turning Nazi propaganda the other way around would mean succumbing to it," believes the researcher. For example, the Expressionist painter Emil Nolde was a Nazi party member and anti-Semite, but he was nevertheless indexed by the Nazis.

Expressionist painter Emil Nolde's Nazi party membership didn't prevent him from landing on the list of undesirable artists

Eighty years after the events, several exhibitions, panel discussions and public debates are being held. Significant headway was made with the discovery in 2012 of the art trove that once belonged to Hildebrand Gurlitt, Hitler's art dealer who traded the banned artworks abroad.

"That was spectacular," says Hoffmann, who also worked as a consultant on the Gurlitt case. "All of a sudden, works reappeared that we long believed to have disappeared."

Along with the exhibition in Dusseldorf and the reworked presentation in Munich, different related shows will be held this year, among others at the Zentrum für verfolgter Künste (Center for persecuted arts) in Solingen and an exhibition of the Gurlitt collection in Bern, Switzerland. Other persecuted artists will also be honored through solo exhibitions, including Rudolf Belling at the Hamburger Bahnhof in Berlin, Max Pechstein at the Bucerius Kunst Forum in Hamburg and Otto Freundlich at the Kunstmuseum Basel.