The story is one of history's big what-ifs. What if the Chinese had persisted, had founded forts, factories and colonies, had developed their technology to keep their lead over the Europeans? Or to put it another way, what went wrong? Why did this tremendous effort subside, with bans on shipbuilding, restrictions on trade, and China sinking back into secretive somnolence? Levathes is, alas, stronger on the picturesque details than she is on these great puzzles. All the colour and curiosity is here, the exotic trade goods, the fascinating personalities.

Interesting stories are cheerfully retold, whether plausible or clearly apocryphal. There are not many written records, but Levathes leaves no anecdote unturned.

Two characters dominate the book. One is the Yongle emperor - a usurper, like most of the more effective holders of the office, and a man with too much blood on his hands for modern tastes.

The other is Zheng He, who commanded nearly all of the expeditions and was clearly a man of remarkable qualities both as a leader and as a mariner. The exertion of these qualities on behalf of the Chinese empire was a considerable act of forgiveness on his part because he was a Mongol, captured during a massacre at the age of 10 and castrated three years later to prepare him for service in the palace.

After a childhood like this we may surmise that Zheng He was no more soft-hearted than Frederick the Great, who also had a bloodstained upbringing. The Chinese expeditions always carried large armies, but the annals are generally silent on what use was made of them.

The last great expedition reached Aden and at least some members of it seem to have journeyed on to Mecca. Zheng He, then 62, was already ailing and died on the way home.

And after this, nothing. A whole tradition and technology were dismantled as a matter of state policy. No satisfactory explanation is offered for this. This hint that it may have been an act of wise forbearance is hardly born out by the general tenor of politics recorded here.

This is not the only part of this book which leaves the reader with more questions than answers. The section on Chinese naval technology is littered with unconvincing passages suggesting, to put it politely, that the author has never set foot in a sailing boat.

More seriously, we are given no real help in assessing the significance of this 30-year burst of activity. Was it an exercise in power, mercantilistic trade, or technological hubris? There are suggestions that the expeditions were launched by taxing provinces for contributions in kind, but the profits, if any, accrued to the Crown. Was the resulting level of taxation sustainable, did the expeditions show a profit? Or was the whole exercise a 15th-century precursor of Concorde - romantically appealing, technically sophisticated and ruinously expensive? The answers to these questions require perhaps, a more sceptical approach than is deployed in this book, which has 3,000-ship fleets and armies of half a million men. The practicalities of these figures are not explored.

But we must not berate Levathes for not writing a book she did not intend to write. This work is a remarkable assembly of information, much of it gathered personally in a huge variety of places.

The story it tells is exciting, dramatic and a monument to the courage and ambition of a heroic age.

There are also some charming tales, which deserve a place in any history worthy of the name. I particularly relished the story of Hu, master of the water clock, who foretold a disaster would befall the new palace in Beijing, stating the day and hour.

The emperor ordered that Hu should be executed at the appointed hour, if nothing happened. As the moment approached, Hu suffered a drastic loss of confidence in his prediction, and took poison.

An hour later lightning struck the palace, which burned to the ground.

Well no, I don't believe it either, but it's a good story.