London has just succumbed to a major invasion by Berlin. Viewed as an invasion plan, however, one can see why whoever did the winning in 1945 managed to scrape home. The smaller bit of the Royal Academy, the Sackler Wing, has been taken over by Botticelli and Dante courtesy, for the most part, of the Kupferstichkabinett - roughly the equivalent of the British Museum's department of prints and drawings. And the Sainsbury Wing of the National Gallery has been occupied by "Spirit of an Age: 19th-century paintings from the Nationalgalerie, Berlin", which consists mostly of German art.
The Botticelli show is modest in size, but gigantic in impact; the National Gallery exhibition is larger and has been significantly oversold. But both shows aesthetically reflect the benefits of the reunification of east and west Berlin, and the establishment of a sensible common cultural policy.
The Nationalgalerie, as the catalogue for "Spirit of an Age" boasts, "is the collection of a nation, in this case of the nation's 19th-century art". Well, yes. One does wonder, though, what Berliners would think if a similar selection of English art from Tate Britain, dating from between 1800 and 1912, were billed as belonging to our National Gallery.
I have read and reread the catalogue, which is well written and informative about the past, and which makes proper mention of the Neue Nationalgalerie for 20th-century art, designed by Mies van der Rohe, and the recent acquisition for the Stulerbau of the spectacular Berggruen modern pictures. Nothing wrong with that as such - except that I can find no mention anywhere in the catalogue of the Gemaldegalerie (literally, the Painting Gallery), one of the world's greatest museums, fit to rank with the Prado, the Uffizi and the National Galleries of London and Washington. A modern architectural masterpiece, opened in 1998 in the "Kulturforum", near to the home of the Berlin Philharmonic, it is by far the most stunning prize of reunification. It gathers together the collections of the Bode Museum from the east and the incomparable Dahlem Museums (think Dulwich, but bigger and better) from the west. It contains, perfectly displayed, room after room of matchless mainstream European masterpieces from the 13th century to about 1800. How odd not to mention it, nor the Kupferstichkabinett. The Nationalgalerie might rejoice in its title, but this is surely a great misnomer, which has served to hype up the National Gallery show without the least justifi-cation. Furthermore, the publicity has laid undue emphasis on the inclusion of "a distinguished group of French impressionist and post-impressionist masterpieces - adventurously acquired at the turn of the 20th century - against the wishes of the highly conservative Kaiser".
Not so. There is one good Cezanne, one competent, conventional Manet, two serviceable but relatively ordinary Monets and one decent Courbet, all of which would not, in my view, exempt the National Gallery's publicists from being nicked under the Trade Descriptions Act.
The catalogue quotes, approvingly, that elegant connoisseur, Harry Graf Kessler: "There is no contrast between being a good German and a good European; a conflict between national and international does not exist." He wrote these resounding words in 1906, just a few years before the two German-activated cataclysms of the 20th century. Nevertheless, the importance of "Spirit of an Age" has been exaggerated to the British public. Presumably the thinking behind all this is that an exhibition called "19th-Century German Painting" (plus a few bits and bobs) won't pull in the punters. All of which is, like any similar cultural spin-doctoring, rather sad as well as superfluous, since it is, on the whole, a well-mounted and fascinating exhibition.
It is almost worth the price of admission just to see the Schinkels and the Friedrichs. But I cannot be the only person to regret the absence of two Friedrich masterpieces owned by the Berlin Gallery, The Monk by the Sea and The Abbey in the Oak Wood, two of the three paintings that Werner Hofmann, in his recent magisterial book Caspar David Friedrich (Thames & Hudson, £39.95), rightly singles out as central works in his oeuvre. There are other, arguably superior treasures in the Berlin Friedrich holdings that have not been brought over. We get the fine landscape The Reisengebirge, but I greatly prefer Morning in the Reisengebirge (known also as The Cross in the Mountains), a magically evocative painting, largely because Friedrich is at his Romantic best when he puts human figures into his mountain scenery - if only to dwarf them into terrified insignificance. Friedrich is the one truly great German artist of the 19th century, so it seems perverse to let us see only seven of his canvases, when there are no fewer than 11 works by Adolph Menzel.
Still, the Friedrichs we do see are a joy. Moonrise over the Sea is a kind of template for Romanticism. Two women, very close together, and a man, somewhat apart, sit with their backs to us on a large rock on the shoreline, contemplating a silvery sea populated by ships in full sail and illuminated by the moon, which is partly rising up through cloud and already making the sea glitter with its descending light. It is painted with a feeling for sea, sky and the subtleties of changing light that Turner himself would have appreciated.
The other Friedrich jewel is his small (34cm x 37cm) Woman at the Window. Painted in his rather austere studio, it is a back view of his wife, Caroline, looking through the open shutters at the boats on the (hidden) River Elbe. The picture and the woman are both neat, economical in design and composition, perfect in proportion, and full of the sense of yearning that is such a vital component of German Romanticism. But the austerity of the studio, and the almost spinsterly look of the tidily put-up hair and slender neck, are not so much mitigated as denied by the explosion of the elaborately pleated dress in a riot of greens, which lights up the entire picture with a beguiling sensuality.
The universal genius of the period - Friedrich was, after all, only a great painter - was Karl-Friedrich Schinkel, a prodigiously gifted stage designer, town planner, architect and, of his day, a painter to rival his almost exact contemporary, Friedrich, when it came to dramatic buildings in even more dramatic landscapes. You see his obviously theatrical qualities, including sumptuous backlighting, in his Gothic Church on a Rock by the Sea and in his stunning composition Medieval City on a River, with an unfinished cathedral, a rainbow and the mysterious city in the background. No wonder the Deutsche Staatsoper Berlin, one of the city's three principal opera houses, staged a production of the Magic Flute last summer with a pastiche of a Schinkel set design.
If "Spirit of an Age" is a mixed blessing from Berlin, there are no such quibbles about the city's other loan to London. Botticelli made his drawings in homage to Dante's Divine Comedy, probably between 1480 and 1495, for Lorenzo de' Medici. Each illustrates one canto, inscribed in black ink on the verso of the large sheets of parchment upon which Botticelli liked to draw. The series was never completed, but 92 sheets survived, to be split up by the middle of the 17th century. Some went to the Vatican, whose eight drawings are here reunited with the other 84, which went to the Hamilton collection in Scotland before ending up in Berlin in 1882. Happily, they survived the war, but in 1945 they were divided, like the city itself, between east and west. Reunification brought these drawings together at the Kupferstichkabinett, and their meticulous display alongside the Vatican drawings at London's Royal Academy is a triumph. It is hard to examine such finely drawn works when surrounded by crowds of people, and one should use this as an excuse to buy the catalogue (priced £27.50), a solidly bound, well-printed volume that is one of the best-looking art books of the past few years. It helpfully reproduces as background many of Botticelli's best paintings, including his curiously modern portrait of Dante, as well as that extraordinary and quite spellbinding panel from Florence Cathedral by Domenico di Michelino - his imaginary version of Dante, crowned with the poet's laurel and opening the Divina Commedia, with the Mount of Purgatory in the centre and the unmistakable profiles of Renaissance Florence's great buildings behind. Michelino's panel summarises perfectly the internal explorations of the drawings, not least because Botticelli borrowed this version of the Mount of Purgatory for his own voyage into the dark wood.
There is something almost numinous in these fragile sheets, more than half a millennium old. The parchment has faded and thinned with time, so that the exquisite calligraphy of the hand- written text on the verso shows through and mingles with, but does not detract from, the drawings created partly by stylus, partly by pen and ink, and partly by pencil. Some of the most haunting sheets are the incomplete ones, where there are small, barely legible pencil sketches, mere ghostly figures, covering as little as a fifth or less of the surface, so that you feel you can actually see Botticelli thinking.
He brings Dante, Beatrice and Virgil fully to humanist life, as they witness every twist of the tortuous and tortured journey. This amazing draughtsman gives you a Lucifer as frightening as any Chinese or Japanese devil, and countless naked human figures, ankles roped and wrists tied behind their backs, waiting for whatever torment is reserved for them.
Botticelli's Chart of Hell shows us Dante's eight concentric circles funnelling down to the ninth and lowest level, with all the detail, and yet economy, of some monstrous Greek vase painting. At the same time, because it is one of the small handful of sheets done in colour, it has all the fiendish skill of a great illuminated manuscript from an earlier time, so it is almost a paradigm of the enormous cultural leap forward that was Renaissance Italy.
"Spirit of an Age: 19th-century paintings from the Nationalgalerie, Berlin" is at the National Gallery, London WC2 (020 7747 2885) until 13 May; "Botticelli's Dante: the drawings for the Divine Comedy" is at the Royal Academy, London W1 (020 7300 8000) until 10 June