Krebs's pictures are just a small portion of the vast number of works of art and artefacts (an estimated two million) removed from Germany by the Soviet Union after the Second World War. Such looting (ordered by Stalin and carried out by battalions of Soviet soldiers, organised into Pythonesque-sounding "Trophy Squads") was a key part of the Russians' revenge on a nation which, as they saw it, had attempted cultural genocide - the Russians had, in the words of the museum's current director, to be "morally compensated".
In the exhibition literature the Russians speak proudly of scholars now being able to examine and evaluate masterpieces to which they had previously attached the suffix "presumed lost in World War II" or "destroyed in 1945". The pictures, they declare, have been "twice saved" - once by the Trophy Squads, who rescued them from the smoking rubble, and once by the conservatoires of Russia's museums. True, in a way, but there is no official explanation of their sudden reappearance, after half a century as "non paintings" in the Hermitage store-rooms. Perhaps the fact that they are on public display, not simply withheld out of spite, will help defuse pressure for their return to their "rightful" owners. The Hermitage has hung 72 oil paintings and two pastels and given them a seductively glossy catalogue, one of whose unwritten functions is to legitimise the presence of the pictures in Russia, presenting them as if they were one homogenous collection. This they are not.
What they are is a gathering together of the best works from seven looted German collections, together with one work - a Monet - plucked from the Kunsthalle in Berlin. So, come on down the heirs of Baron von der Heydt, Alice Meyer, Otto Gerstenberg, Bernhard Koehler, Friedrich Siemens, Monica Sachse and, last but not least, Otto Krebs. These are your pictures. Or are they? The Russians, understandably, don't agree. A committee of curators, government officials and, above all, lawyers is even now attempting to decide the ownership and eventual fate of the paintings. But what exactly is at stake?
The first reaction to this exhibition is astonishment. Our taste for the Impressionists is jaded by over-familiarity - so to see a work whose authorship is immediately evident and not to recognise the painting itself comes as something of a revelation. The show includes, for example, Rocks, a work by a fore-runner of Impressionism, Corot, which is not included in Daulte's catalogue raisonne, nor in any subsequent literature. This is a beautiful, fascinating and vitally important new addition to Corot's oeuvre - and my distrust of the catalogued date, 1828, is just a taster of the kind of art-historical in-fighting this exhibition will unleash.
But this is merely a taster for the main attraction, the Impressionists. Taken as a single collection, the works in the Hermitage demand comparison with the museums of Albert C Barnes and Samuel Courtauld. While the Russians win hands down on Fantin Latour still lifes, the Hermitage's two Manet portraits are largely unprepossessing. But Degas's Place de la Concorde (1873) emerges as one of the seminal works of early Impressionism. Here is the very first attempt on canvas to capture the urban experience of the street.
The group of 15 works by Renoir, drawn from the Gerstenberg, Koehler and Krebs collections, proves an even greater eye-opener. The Courtauld has six major Renoirs, the Barnes no less than 16. With a few exceptions every one of these is an important, finely executed tour de force. No such description can be applied to the works in the Hermitage. In the Garden (1885) and Young Woman in a Floral Hat (1892) are two of the worst paintings that I have ever seen. Young Girls at the Piano possesses none of the delicacy of touch or compositional harmony of the version in the Orangerie. The rest (save perhaps one semi-nude) are no better. If these really are by Renoir (and none of the attributions seem to be in doubt), then we must re-evaluate our opinion of him as a great master.
But the Hermitage's six Monets are a quite different story. All save two originate from the Krebs collection, whose eye for Monet was clearly better than it was for Renoir. Take The Seine at Rouen, (1872) for example, a work sketched quickly and lightly on to an unprimed canvas. What is so exciting is to find him using a tight compositional framework (pointing to the influence of the Japanese print) so early in his career. Abandoning conventional perspective, he defines his space with colour, light and shade. He even uses the rippling lines of orange upon the water that so memorably define Impression, Sunrise of 1873, ostensibly the first "Impressionist" painting. It seems appropriate that it should hang beside The Grand Quai at Le Havre, which itself provided the counterpoint for Impression, Sunrise at the first Impressionist exhibition in 1874.
That Krebs bought with such sensitive juxtapositions in mind becomes clear as the exhibition unfolds. His five paintings by Pissarro fill, as a group, the gap evident in both the Courtauld and Barnes collections. It was Krebs, too, who owned the five best of the show's seven Czannes and all four of its Gauguins. It is, however, with a magnificent group of works by Van Gogh that the collector really makes his mark. Here are four classic images, all painted in 1889 and 1890 - the artist's crucial last two years. The Landscape with House and Ploughman, in particular, possesses the power to reawaken our eyes to the dynamic boldness of Van Gogh's achievement.
The least the Russians could do now would be to hang the Krebs Collection on its own and give it a catalogue - it is quite the equal of that bequeathed by his contemporary Samuel Courtauld to the British nation. Given Krebs's reputation for philanthropy, it seems not unlikely that, had events gone otherwise, he might have done the same service to his own homeland. But for Russia to return these priceless pictures would be an unthinkable act of clemency from a money-hungry nation still grieving for 22 million war dead.
These pictures have become more than paintings - they are the talismans of a nation's grief and glory. We shall have to wait for the lawyers and the governments to have their say, before these unfortunate masterpieces can find their final resting place, and ceasing to be the currency of conquest and revenge can become, once again, works of art.