It is now more than 30 years since a peasant digging a well in northwestern China unexpectedly came across some fragments of fired clay, triggering the greatest archeological discovery of the 20th century – the terracotta army guarding the tomb of China’s First Emperor. The soldiers quickly became international celebrities, travelling in twos and threes to exhibitions across the world. Since 1974, excavation of the tomb has continued steadily. Slowly – infuriatingly slowly for an impatient world – a huge underground complex began to emerge. This was not just an army for the afterlife, but a parallel kingdom. While the work will carry on for decades, every few years to date have yielded major discoveries. And at each step, our assumptions about the scale of the emperor’s ambitions have had to be revised, as the warriors have been joined by more and more departments of the vast subterranean imperial household.

And so, as well as the original foot soldiers, we can now see emerging other key parts of an effective fighting army: generals in full dress uniform, standing and kneeling archers, chariots and charioteers, horses, stableboys and cavalrymen. In a fascinating mix of reality and fantasy, we have found an arsenal of impractical stone armour, yet also bales of real hay. Armies, of course, require administration, and a whole pit of bureaucrats was unearthed in 2000 – perhaps not surprising in the country that invented the idea of an official civil service. Less expected were the acrobats discovered a year earlier, presumably to perform at posthumous imperial variety shows. Andin 2001, excavators came across a group of terracotta musicians in a pit, seated by an underground stream completewith magnificent, life-size bronze birds who were perhapsdancing to their music for the perpetual delight of theemperor. The afterlife was to be just as beguiling as this one.

Examples of all these new discoveries are included in the First Emperor exhibition, which opens at the British Museum next month. The objects are so important that they travelled to Britain in four planes to minimise risk. Along with more than 100 other loans, there are more than 20 full-scale figures, the greatest number the Chinese authorities have ever allowed to travel to an exhibition overseas, and an act of outstanding generosity to the UK.

It is the measured, decades-long campaign of excavation and the steady flow of important new material that make this find so entirely different from the two other archeological sensations of the last century – Tutankhamun and Sutton Hoo. Tutankhamun’s tomb, once discovered, was swiftly emptied, and its contents were removed to Cairo. Sutton Hoo’s Anglo-Saxon ship burial had to be unearthed in one go, in a frantic race against the onset of war in 1939. In each case, we knew almost at once what had been found. But with the First Emperor’s tomb, we are still only at the beginning of excavating a site the size of the city of Cambridge, and each discovery inflects the interpretation both of what has already been unearthed and of the man who still lies buried at its heart. With leading international scholars proposing their interpretations of the new discoveries, this exhibition presents not so much conclusions but research, a continuing process of revelation. History in the process of being written.

It is a history the world needs to know. This tomb complex is the richest source of evidence about a man who unquestionably changed the world, and in a way that still materially affects us today. Between the years 240BC and 220BC, as king of Qin, a small, warlike state in northwestern China, he conquered the neighbouring kingdoms to the east and south, combining them into a single empire and taking the title Qin Shihuangdi, the First August Divine Emperor. To hold his new state together and make it an economic unit, he unified laws, standardised weights and measures, and introduced a single currency. These reforms were astonishingly long-lived. His new coin type, round with a square hole, remained in use until the 1950s and is still the logo of the Bank of China. Above all, to overcome the huge linguistic differences in his new empire, he imposed the universal use of a standard script – essentially the characters still in use now. It is hardly an exaggeration to say that this is the man who made China – the oldest surviving political entity in the world. To understand how modern China came to be what it is, the First Emperor, and his tomb, are a key part of the story.

How did he do it? How did he unify the warring states and build the largest, most complex tomb yet discovered on earth? Generalship and high political skills, certainly. Administrative brilliance, without doubt. And the brutal coercion of thousands. But if you were to suggest one practical element that made possible both his military triumph in life and the enormous parallel empire built underground for the afterlife, it would have to be a phenomenon widely associated with modern China: mass production.

In the exhibition, we will show some of the standardised crossbows, produced in huge numbers – the Maxim guns of their day – that helped give Qin archers their victories. And we can show that the terracotta figures were the result of a honed system of production lines, organised initially to produce roof tiles and drainpipes for the emperor’s new palaces. Quality-control stamps can still be seen on the backs of some of the standing figures, with certain foremen’s names also occurring on the royal roof tiles. The terracotta figures are, in fact, mix-and-match assemblages of moulded parts made in huge numbers.

But they are finished individually and carefully turned into self-standing sculptures, minutely differentiated in costume and facial type, so that role, rank and place of origin can be clearly read. As you look closely at the warriors, you see that, far from being a homogeneous group of men, they are soldiers from different regions, with very different facial features. This army clearly comprises many diverse peoples held together for a common end. We are looking at the portrait of a whole empire.

It is this kind of close-up examination that we hope the exhibition will make possible. Obviously, we cannot hope to replicate the experience of the massed ranks of the excavated pits. But the visitor to Xi’an cannot easily engage closely with the sculptures, look them in the eye (some stand a startling 1.95m tall), admire the skill with which each one has been finished or spot the traces of pigment which reveal that this entire army was once riotously coloured, decked in purple and green, orange and white.

An exhibition like this lies, I believe, at the heart of the British Museum’s purpose. Set up in 1753, when the world was experiencing the first wave of economic globalisation, it was designed to allow the public (even then defined as a world public) to make sense of a new world order. Today, London is once again at the centre of a transformed – and transforming – global economy; and now, as then, we need new histories to make sense of our new present. Very few of us were taught the history we need to understand modern Asia – or, indeed, most of the world beyond Europe. An exhibition like this allows us to think about China in the only way that might let us understand one of its central elements: its immensely long and complex history. It is part of what the British Museum is for.

And so I feel it is appropriate it should be displayed at the very heart of the Museum’s buildings. We have transformed the Round Reading Room. Because all the desks and chairs are listed, we have built a vast steel platform over them. The great domed cylinder provides, I believe, a worthy setting for an emperor’s tomb. Visitors will form their own view of the evocative power of the space, but I do hope some will think of the desks below, where 150 years ago Karl Marx pondered the fate of empires and of the world economy, the mysteries of shifting power.

This exhibition is part of a five-year programme of exchanges between the British Museum and museums in China. Earlier this year, we organised, along with Chinese curators, an exhibition in the Forbidden City. Here, a selection of objects from the British Museum addressed a very particular historical issue – Britain’s rise from regional to world power during the 18th century. It was a topic requested by our Beijing colleagues, and one that stimulated lively public discussion, most notably on television, of the parallels between Britain then and China now. What lessons should China learn from Britain’s economic history?

These are the kind of exchanges to which the Chinese attach great importance. It allows their public to explore other cultures – Chinese museums essentially hold only Chinese material – and London visitors can discover aspects of Chinese history not represented in any European collection. So the British Museum/China exchange programme was endorsed at the highest level, signed with pomp in Beijing’s Great Hall of the People in the presence of both countries’ prime ministers.

But political support, although invaluable, would not be enough to produce exhibitions of this sort. For decades, the curators and conservators of the British Museum have been working with their Chinese colleagues, researching together both the history and the conservation of the objects in their charge. It is this sort of long-running friendship, and the building of an international community of inquiry, that makes such scholarly, and popular, exhibitions possible. They are, I believe, a modern version of the Enlightenment dream – a global conversation in which millions of citizens can take part. We hope hundreds of thousands of them will come to the exhibition.

The First Emperor: China’s Terracotta Army is at the British Museum, WC1, from Sept 13 until April 6, 2008; sponsored by Morgan Stanley. Bookings on 020 7323 8181 or at www.thebritishmuseum.ac.uk/firstemperor

To view pictures of the terracotta warriors, visit timesonline.co.uk/visualarts

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