Just south of the palatial grounds of the British embassy in Tehran stand two of the world's more unmissable museums. The first is devoted to the arts of the Muslim period: from calligraphy to carpets, Iran's Islamic heritage is to be found there, lovingly and expensively displayed. The best way to appreciate just how many resources have been lavished upon this modern and airy shrine to the splendours of Muslim Iran is to cross a forecourt and visit its twin. The National Museum is a gloomy mausoleum that showcases a much earlier period of history: the long millennia before the arrival of Islam. The treasures here are among the most archaeologically significant in the whole of the Middle East - but their setting is dingy and depressing. Many of the most precious artefacts are not even on display, but have to be kept locked away in a basement because of security inadequacies. Gold and silver are notable by their absence. The display cabinets incline instead to a dull grey monochrome.
This neglect of the country's pre-Muslim past is unsurprising. Iran is an Islamic republic and it can often be tricky in the Middle East to divorce ancient history from modern politics. An ayatollah would look upon the contents of the National Museum with a jaundiced eye. The most splendid exhibits of all date from the 6th and 5th centuries BC, a period when the Persians of Iran ruled an empire that stretched from the Aegean to the Hindu Kush, and were themselves the subjects of a "King of Kings" - a "Shah an Shah". The Pahlavi dynasty, keen to identify itself with such A-list predecessors, exploited the glories of the Persian empire with shameless gusto. When the last shah was toppled in the 1979 revolution, Ayatollah Khomeini condemned the millennia-old traditions of Persian kingship as explicitly un-Islamic. One excitable mullah went so far as to suggest that Persepolis, the best preserved of all the ancient palaces, be bulldozed to the ground.
But the revolutionary government, even at its most militantly theocratic, would never have countenanced such vandalism. The ayatollahs are no Taliban, and most Iranians, whose sense of national identity has survived countless upheavals, retain a deep sense of pride in ancestors who successfully forged history's first world empire. Now, more than 25 years after the Islamic revolution, the Iranian government has finally shown itself willing to follow in the shah's footsteps and capitalise upon the glories of its country's ancient past for diplomatic ends. Startlingly, it is we in Britain, the erstwhile Little Satan, who find ourselves the beneficiaries.
We owe this not merely to the relative thaw in Anglo-Iranian relations, but also to the ambition and persuasiveness of John Curtis, curator of the Ancient Near East department at the British Museum, who has spent two decades pushing for an exhibition devoted to ancient Persia. The result is a spectacular coup: a show that brings together for the first time the most significant Persian antiquities from the British Museum and the Louvre, as well as from Persepolis and Tehran. Thrillingly, even some of the treasures normally kept in deep-storage beneath the National Museum have been disinterred.
Which is not to say, even now, that the vaults of Tehran have been emptied, that it is necessarily all precious metals and bejewelled magnificence. The title that Curtis has given to the exhibition, "The Forgotten Empire", reflects the undoubted fact that evidence of how ancient Persia functioned is often fragmentary and diffuse. Yet it is precisely this that makes the exhibition such a revelation. Over the past 30 years, the recovery of ancient Persia from oblivion has been one of the great success stories of ancient history: a whole empire has been brought back to life, rendered so solid that it has become, in the words of one historian, "something you can stub your toe on". Previously, evidence of this breakthrough was confined to scholarly tomes and journals: now, with the exhibition at the British Museum, the general public can share in a display of resurrectionism so remarkable that, while it may not provide us with golden death masks or sarcophagi, it is certainly fit to rank beside the opening of Tutankhamun's tomb.
Indeed, the true brilliance of this exhibition is the way in which it displays works of art that are variously stupefying and exquisite, simultaneously dramatising the ambiguities that must always affect the study of the ancient past.
Pass through the first two rooms and the characteristic of Persian imperialism that most appealed to the last shah, and so repelled the ayatollah, strikes the visitor like a hammer blow. "I am Darius the Great King, the King of Kings, the King of Lands": so boasted Persia's greatest monarch, Darius I, on a silver foundation plaque from Persepolis. The artefact is so priceless that it has never before been seen in a museum display case and, indeed, under the shah, was used to underwrite the Iranian national debt. The same message of royal greatness is evident everywhere: in a statue of Darius, in the scale of a column base from his palace at Persepolis, in casts (made for the British Museum back in the 19th century) of provincials bringing tribute to their master.
And yet these same reliefs, even as they affirm the unprecedented power of the royal centre, also speak of something else: of a dominion so vast in scale as to defy rigid centralisation. Move on around the exhibition, into rooms devoted to the functioning of the empire, and the sense of any monolithic imperial culture immediately starts to dissolve. The art of ancient Persia derives from wellsprings that are often far removed from the heartlands of Iran: Greece, Egypt, the central Asian steppes. Look carefully at an amphora found in Persepolis, for instance, and one can just make out, stamped on the rim, the delicate image of a trireme, the brand of a city in distant Lebanon. To describe the artefact merely as "Persian" raises any number of questions - some of them surprisingly modern. Does a superpower best maintain itself by crushing or assimilating what is foreign? Indeed, how much sense does it make to talk of national identity at all in an era of globalisation?
This exhibition gives no easy answers, and is all the better for it. Museums, by their very nature, can often convey a sense of ancient civilisations as things static and frozen, reducible to artefacts placed in cabinets, the equivalents of dead butterflies pinned to a tray. This show, better than any I have seen, serves to convey the true excitement of ancient history: that it is concerned with people just as much afflicted by imponderables and ambiguities as we are today.
Forgotten empire? Maybe. But ancient Persia, thanks to this exhibition, should soon be a good deal less forgotten than it was.
"Forgotten Empire: the world of ancient Persia" is at the British Museum (020 7323 8299) until 8 January 2006
Tom Holland's book on ancient Persia, Persian Fire, is published by Little, Brown (£20 hbk)