Through it all, crooked experts and entrepreneurs managed to exploit the situation, often smuggling artworks out of France or Belgium or Holland into neutral Spain and Portugal. Here they were usually stored in warehouses or in free ports, before being sold off privately or shipped overseas, and a considerable number of stolen or cheaply acquired masterpieces turned up on the US market, especially in New York. South America was another potential market, particularly Argentina where Goering is alleged to have sold off much of his loot. Later, after the German defeat, American soldiers smuggled back home works they had stolen or bullied from their owners in Germany and Austria, just as Russian soldiers took home with them their private loot.
There are some heartening chapters in this grim story, however. One concerns the famous salt-mine at Alt Aussee in Austria, where a huge gathering of art treasures - including many of Europe's greatest paintings and sculptures - had been stored secretly on Hitler's personal orders. With the Allied armies advancing, and Allied bombers devastating whole cities and towns, it was considered a safe hiding place. However, as they drew nearer, Hitler decided on a typical Gotterdammerung gesture and placed a fanatical Nazi named Eigruber in charge of the mine, with orders to plant bombs and explosives and blow it all up rather than surrender it to American or British troops.
Anti-Nazi groups learned of this and tipped off English intelligence in London, who organised a counter-stroke. Four native Austrians, headed by a Leftist sympathiser named Albrecht Gaiswinkler who had defected to the Allies, were specially trained and parachuted into the area. Narrowly escaping military patrols, they got in touch with the local Resistance, subverted some wavering Nazi officials, and managed to infiltrate the mine and save it from destruction.
These remarkable men fought off military attempts to regain it and even attacked retreating German units. In the postwar years, however, they received neither medals nor pensions and became forgotten men officially, in spite of rendering European culture an outstanding service.
There were other heroes too, including the great archaeologist Leonard Woolley, then an officer in the British Army, who worked unsparingly to save treasures from looting or destruction and had a major part in limiting the mass bombing of Italian historic buildings by the American air force. Certain cultured Americans, too, played a positive role.
Perhaps 80 per cent of what vanished has been recovered over the years, although many masterpieces remain missing including a Raphael painting taken from the collection of Count Zamoyski in Poland.
Much remains unclaimed, mostly the property of Jewish families who vanished in the Holocaust. Other Jews were so heartsick that they made no claims in the years immediately after the war, leaving grandchildren to sue for the return of family possessions and heirlooms. Even when the works they legally claim are housed openly in public galleries and collections, theirs has been an uphill battle, involving high legal costs and relatively few satisfactory settlements. Gradually, international control and international awareness of the situation in all its complexity has grown, although Russia remains recalcitrant about what it seized in Germany and Austria. Austria itself, which for decades had rebuffed suggestions that it should return purloined artworks to their original owners or their descendants, has recently adopted a much more positive approach.
Multi-national conferences have taken place, involving top art scholars and administrators, while influential Jewish groups have exerted steady and vocal pressure. Switzerland, whose bank vaults were believed - in fact, known - to hold huge amounts of gold and treasures secreted there by Jews in the years before the Holocaust, has finally agreed to make a cash settlement. Meanwhile, the documentation continues to pile up and more and more claims come before the courts of many countries, including the US.
Plainly, it will take decades yet before the legacy of war and persecution is sorted out and justice can be seen to be done.
The Lost Masters: the Looting of Europe's Treasurehouses is published by Gollancz at £20 in UK