In the summer of 1939, as Europe lurched ever closer towards war, the seemingly unstoppable rise of Adolf Hitler led to an exodus of French Jewry as thousands of families frantically packed up their belongings to flee the inevitable Nazi invasion. As he hurriedly planned his own escape, a young Jewish art dealer, Erich Slomovich, walked into a bank vault in Paris and hid a collection of paintings in the vain hope that he and the artworks would somehow survive the conflict.
His boss, the legendary French art collector Ambroise Vollard, had recently died in a car crash, leaving his Croatian-born assistant in possession of a string of Modernist masterpieces including works by André Derain, Henri Matisse, Paul Cézanne and Pablo Picasso.
Once the paintings were safely stowed away in the vaults of Société Générale, Slomovich fled to his native Yugoslavia but was eventually caught by the Germans and sent to the gas chambers. For four decades, the remarkable horde he had helped to protect from Nazi looters gathered dust inside the vault, although rumours of Vollard's lost collections had long been heard among the art-dealing fraternity.
When officials finally opened the vault in 1989 and discovered what was inside, it sparked a monumental series of court battles as the Vollard and Slomovich families battled it out with competing claims to the artistic treasure trove. Now, more than 70 years after it was first hidden, the remarkable collection is to be sold in one of the biggest Modernist auctions of the year – the first time many of the works have been seen in public since the Second World War.
They include a selection of prints and etches from some of France's most influential modern painters including Cézanne, Degas, Gaugin, Renoir and Mary Casatt. The two sales, hosted by Sotheby's in London and Paris in June, are expected to realise up to $26m (£16.9m).
The lot with the largest price tag is a canvas painting by Derain, who spearheaded the short-lived Fauvist movement with his contemporary Henri Matisse. The pair used such bold colours in their works that they were quickly labelled "les fauves", or "wild beasts", by critics of the time, who were scandalised and enthralled in equal measure.
Arbres á Collioure, a woodland landscape painted by Derain in southern France in 1905, has an estimated sale price of £9m to £14m, which would smash the previous record for a Derain painting, set last year in New York at £8.5m. Georgina Adams, the market editor-at-large for The Art Newspaper, said she believed Sotheby's estimate was a little high, but added that collectors would often pay more for paintings with interesting stories behind them.
"Derain is a very uneven painter who did some real turkeys, but his Fauve works are highly sought-after," she said. "This is a remarkable and exciting painting because it has never been on the market before and it has a fascinating back-story. That will always increase buyer interest."
Because of his untimely death, little is known of how Slomovich actually managed to get his hands on so much of Vollard's collection. Just before the war broke out, Vollard's chauffeur-driven car skidded off the road, killing him and his driver. There have long been rumours that Vollard, then 73, was murdered by a Corsican mafia boss. As war approached, Slomovich in charge of hiding the Vollard collection from the Nazis.
In legal battles in the 1980s and 1990s, Slomovich's heirs claimed Vollard gave the paintings to the young entrepreneur so he could launch a gallery in Belgrade. Many books and pictures in the collection were personally inscribed by Vollard, who also wrote letters of introduction for Slomovich to artists such as Pierre Bonnard and Georges Rouault.
But Vollard's family argued that he gave the works to his business partner to sell on his behalf. The court cases dragged on for decades but were eventually settled in 2006, with the most of the art collection going to Vollard's descendants. A much larger portion of the collection was taken by Slomovich to Yugoslavia, where he had hoped to set up an exhibition at the National Museum of Belgrade, in what is now Serbia. He was forced to flee once more when German forces invaded, hiding out in a village near the Yugoslav capital. Eventually, Slomovich and his family were betrayed to the Nazis by locals and were packed off to a concentration camp. It is believed he was killed in a portable gas chamber in the summer of 1942. He was 27 years old.
The paintings he had managed to take with him to Yugoslavia were successfully hidden by his relatives during the war, but were promptly seized by Marshal Tito's communists and "gifted" to the Yugoslav state. More than 125 pieces, including a string of works by Degas and Renoir, are held by what is now the National Museum of Serbia. The works being sold by Sotheby's are those that ended up in the hands of Vollard's heirs.
* During the Second World War, the Nazis perfected the art of looting with meticulous planning and ideological zeal.
* Adolf Hitler was obsessed with art and was determined to build the world's largest museum in his home town of Linz, Austria.
* The "Führermuseum", as it came to be known, was never built but that did not stop the Nazis plundering vast amounts of paintings, books, artefacts and treasures.
* Collections owned by Jewish families were most at risk, particularly if they contained the kind of classical masterpieces that Hitler and his fellow art enthusiast, Hermann Goering, admired. Both regarded modern art as an abomination.
* It is difficult to estimate how many works were stolen by German troops. The US National Archives has suggested that up to one fifth of the art in Europe might have been appropriated by the Nazi war machine.
* The allies found stolen artworks in more than 1,050 repositories in Germany and Austria at the end of the war. Much of it had been hidden in salt mines to protect it from regular allied bombing raids that began when the Luftwaffe lost control of German airspace.
* Although much of the loot was returned to its rightful owners after the ended in 1945, hundreds of thousands of artefacts are still missing. Only in 1998 did 44 leading nations finally commit to searching for artworks plundered by the Nazis and to return them to their owners.