Why do we find mysteries so fascinating? Perhaps, to solve a mystery is to prove to ourselves our own mastery of life—that we can figure out the answer, and in so doing, defeat a nefarious purpose and restore order and right to the world.

If solving a mystery is seeking justice, then the ongoing mysteries posed by Hitler, his henchmen and the Third Reich are at the top of our attention. How did they escape? Are they still amongst us, or hiding in the jungles of Paraguay? Even today, seventy-five years later, we’re still pursuing Nazi war criminals.

Given the passing of time and the aging of the perpetrators, that long hunt will soon end. But the legacy of one Nazi mystery will remain—we still don’t know what happened to some of Europe’s great artistic treasures that vanished in the war, like the famous Amber Chamber or Raphael’s self-portrait. We’re still coping with the chaos and disruption that the Nazis brought to Europe’s rich artistic heritage, and sorting out their many stolen treasures. When I was researching my novel New York Station, which is based on the true story of the little-known attempt by Nazi Germany to rig the 1940 Presidential election, I kept running into accounts of these thefts. It fascinated me and I knew I had to write about it, and that became part of the next book in the Roy Hawkins’ series, The Hungry Blade.

Anti-semitism, xenophobia and nationalism, along with a carefully cultivated mythos of victimization, lay at the core of Nazism. For all of that, the Nazis were also particularly preoccupied with art. Much came from Hitler himself.

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As a young man Adolf Hitler had been an aspiring artist in Vienna, and many of his paintings have survived. Technically, most are quite competent. At that time the art world was already bubbling with an unprecedented explosion of creativity: Expressionism, Fauvism, Cubism, Constructivism, Surrealism, all began before World War One when Hitler was painting.

But Hitler’s paintings were conservative and representational. They have an eerily cold and sterile quality, especially street scenes and architecture. There’s little emotion or feeling in them, including a very odd Madonna and child—featuring a baby Jesus with strawberry blond hair. In that lack of emotion or empathy they may represent an insight into the mind of a psychopath.

Hitler had been rejected by the Academy of Fine Arts in Vienna twice when the First World War and military service intervened and he never resumed his artistic endeavors, turning to politics instead. How he felt about this rejection may have fueled a hatred of modern art, which he expressly raged against in his book Mein Kampf. To Hitler, modern art was both a degenerate product and symbol of a decadent 20th century society he hated and wanted to destroy.

Hitler saw himself as an art expert and when he seized power he wasted little time imposing an officially endorsed “realist” art on Germany that was as cold and sterile as his own. He promptly purged Germany’s museums. 16,000 pieces were “deaccessioned.” Some were thrown on pyres in the streets with another 650 placed in an exhibit titled Degenerate Art in 1937.

the Nazis’ greed and ambitions collided with their desire to wipe out modern art. They hated it, but it was also like creating a bonfire out of cash in the middle of the street.

All were marked for destruction. Then the Nazis’ greed and ambitions collided with their desire to wipe out modern art. They hated it, but it was also like creating a bonfire out of cash in the middle of the street. Hitler wanted to create a museum named after him in Linz, and Reichsmarshall Göring quickly followed suit. Instead of burning the works they decided they would sell them to finance these museums. They began selling the seized artworks in whatever channel they could, particularly through Switzerland. Before the war many works headed to the United States and New York, the world’s richest art market. They dumped so many they actually depressed prices for a time.

That initial theft was the entering wedge. For all their crimes—and nothing can compare to the Holocaust or the deaths of millions—how just plain crooked, corrupt and dishonest the Nazis were often gets overlooked.

After looting the German state museums, they next went after Jewish collections. German Jews who were forced to flee had their property confiscated. Once the war started the Nazis really got going with stolen loot, beginning what can best be described as a program to strip mine conquered countries of their cultural treasures and ship them back to Germany.

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Göring led the way. After the German occupation of the Continent the Galerie Nationale du Jeu de Paume in Paris was taken over by the Reichsleiter Rosenberg Taskforce of the Nazi Party and used as a central storage and processing depot for tens of thousands of looted art works. Between 1940 and 1942 Göring personally made twenty trips to Paris alone to select almost six hundred old masters for his personal collection. His first shipment of stolen works was enough to fill thirty boxcars.

The Nazis still wanted to sell the “degenerate art” abroad, partly to line their pockets, but also to finance the Nazi war effort. However, once the war started that became more difficult, especially after the fall of France in June 1940 and Fascist Italy’s late entry into the conflict. The United States, in the grip of isolationism, was initially determined to maintain its neutrality. Equally bad, for the purposes of fencing stolen art, both Britain and Germany declared blockades of each other, partly closing the Atlantic and Mediterranean oceans, radically reducing the number of cooperative neutral countries.

Looted artworks subsequently went all over the world, which is why so many have yet to be recovered to this day. (In my story, The Hungry Blade, art works looted by the Nazis are routed through Mexico to evade the US Neutrality Laws.)

The Nazis routinely acted through “cut-outs,” existing art dealers who unscrupulously fenced the art works, giving them at least superficially clean titles of ownership and provenances. They often mixed Nazi loot with legitimately purchased or consigned art works, making it even harder to separate stolen works from normal art commerce.

One example was German art dealer Hildebrand Gurlitt. After WWI Gurlitt became the director of the König Albert museum in Zwickau and then the Kunstverein in Hamburg. At both museums he organized exhibits of often controversial modern artists, until he was fired by the Nazis. Gurlitt then became a dealer. Gurlitt bought art from fleeing Jews at fire sale prices, making a tidy profit. Gurlitt was then appointed to a Nazi Commission for the Exploitation of Degenerate Art and tasked with marketing confiscated works of art outside Germany. At the same time Gurlitt was also legitimately buying and selling art. Thus, his account books contained works that were stolen outright, works of dubious legitimacy because the owners effectively sold under duress, and honestly purchased pieces. That was perfect for the Nazis’ purposes—it was extremely difficult for anyone to determine what was what, a problem that exists to this day.

The US Army took Gurlitt into custody after the war, and temporarily confiscated works he then held. Gurlitt claimed most of the art along with his records, some 1,500 pieces, were destroyed in an Allied bombing raid. The Army was unable to prove anything and he was eventually released with his remaining collection returned to him. He went back to being an art dealer, director and promoting modern art, and died in 1956.

A startling development came in 2012 when German authorities discovered his son had a cache of almost 1,500 pieces hidden away in an apartment in Munich: room after room packed wall to wall with paintings, many of them by top masters. The German authorities seized the collection, which the son wanted to bequeath to a museum in Switzerland. But separating stolen pieces from distress sales and legitimately acquired works has proven to be an arduous task that will probably continue for years to come.

The US and Royal Armies made extraordinary efforts to find these stolen works and return them to their rightful owners, which was featured in the 2014 film The Monuments Men (unfortunately, the movie seriously slights the British effort—the Royal Army’s program began well before the American one and Britain was a full partner in this project.)

Miraculously, most of the stolen art works that passed through the Jeu de Paume in Paris– 60,000 some pieces—were eventually recovered thanks to the incredible bravery of a single French woman, Rose Valland. Valland became its overseer during the Occupation. Unknown to the Germans, Valland understood German, which she never let on. Partly by hanging around and simply listening to the Germans talking, Valland was able to keep track of an extraordinary number of art works, and also where they were being shipped to in Germany. At considerable risk to herself, she kept a careful ledger of what went through the Jeu de Paume.

But that was hardly the end of the chances she took: Valland also established contact with the French resistance and alerted them not to blow up trains carrying away France’s artistic heritage.

As if those risks were not enough, Valland was a lesbian, and had to stay closeted—the Nazis systematically sent LGBT people to concentration camps for extermination. (The Monuments Men movie got her orientation wrong as well—the character based on her is depicted as being attracted to an American officer.) Valland spent eight years in the French Army as a captain after the war, discovered several major Nazi hiding places, and testified against Göring at the Nuremberg War Crimes trials.

Unfortunately, other countries were not as lucky. One of the most famous rooms in Europe was the Amber Chamber. A gift from the Prussian King Frederick William I to Tsar Peter the Great, the Amber Chamber was constructed in the Baroque style completely of amber, precious gems and gold. The Nazis looted it in 1941 and moved it to Germany. It probably was destroyed by a combination of an RAF bombing and Soviet artillery at the end of the war, but rumors persist it was hidden away, and there are periodic searches for it.

The greatest missing work and enduring mystery is what is believed to be Raphael’s self-portrait. It was purchased by Prince Adam Jerzy Czartoryski in Italy in 1798 and brought back to his castle in Poland. The portrait—along with other works taken from a hiding place after the invasion of Poland in 1939—eventually ended in the custody of Hans Frank, the Nazi governor of Germany’s General Government of Poland. Fleeing the advancing Red Army, Frank took his collection with him to Silesia. It, too, may have been destroyed in a bombing raid but the Polish government claims it knows the painting survived the war and presumably is in someone’s collection somewhere, with periodic reports of clues to its whereabouts.

It’s possible. Gustav Klimt’s Portrait of Adele Bloch-Bauer was discovered to have been illegally sold and was finally returned to its rightful owners in 2012 after a legal case that went to the United States Supreme Court. The mystery endures. If the art works were not destroyed, in time more will be recovered.