ROUND 500 years ago, our hitherto slowly altering world began to change, and in amazingly swift ways -- ways that have affected us all, and make it impossible ever to go back. From a small and rather miserable peninsula -- an area commonly known as Christendom or Europe -- at the southwest corner of the gigantic Eurasian landmass, men began to venture forth in frail yet efficient wooden sailing vessels across thousands of miles of ocean. Carried westward and southward by winds and currents, they discovered what they came to term the ''new world,'' although it turned out to be many new worlds.

While this development is commonplace in all our history books, and was recalled in many ways at the quincentennial celebrations a decade ago, it is important to note how extraordinary it was. This was not an adventure undertaken by a fleet of Indonesian ships arriving off the Scottish coast in 1500. Nor by a Zulu flotilla bringing an army of conquest to Maine. It consisted of a small collection of rash, visionary and often fearful West Europeans going forth to catalyze the globe. Today, billions of people, descendants of the venturing nations and descendants of those who were invaded, still stand in the historical shadow of this epic transformation.

Many generations of historians have attempted to explain the reasons for Europe's amazing rise to world power. Was it due to its move toward rationality and science during the Renaissance, or its capacity for organization, or the competitiveness of its nation-states (as opposed to the dull uniformity of Oriental empires), or its favorable geographical position, or its gunpowder revolution?

Probably it was due to all of them, a sort of fusion of historical forces. But it also needed something else: human ambition. It required people willing to risk all in pursuit of power, wealth, glory and divine approval. And it is this human ambitiousness that is at the core of Hugh Thomas's magisterial and sprawling new book, ''Rivers of Gold: The Rise of the Spanish Empire, From Columbus to Magellan.'' Here is a work that seeks not so much to explain the backdrop to early European imperialism as to describe for modern readers the visions and sufferings of the driving personalities who accomplished the conquest of so much of the globe in so short a time.

The focus is upon Spain, and rightly so. The Portuguese may have been the first Europeans around Africa; the Dutch and French may later have implanted themselves around the Indian Ocean; and the British may have brushed them all away, in the 18th-century wars of the Elder and Younger Pitt. But it was the Spanish explorers and conquistadors who set the pace and tone, not only in the Western Hemisphere but also in the Pacific.

A book the size of ''Rivers of Gold'' would be an astonishing work by any author, yet its publication simply affirms Hugh Thomas's record as one of the most productive and wide-ranging historians of modern times. Born in 1931, for many years a professor of history at the University of Reading and made Lord Thomas by no less than Margaret Thatcher in 1981, he first caught public attention with an enormous tome, ''The Spanish Civil War,'' in 1961. Ten years later he released his vast study ''Cuba: The Pursuit of Freedom.'' Then came his great ''Unfinished History of the World'' (1979). In 1997 he published ''The Slave Trade.'' But Thomas has also written about European unity and about the British radical John Strachey. Ten years ago he published another large work, ''Conquest: Montezuma, Cortes, and the Fall of Old Mexico,'' which is the closest in theme to the present volume.

''Rivers of Gold'' takes just about 700 pages to describe only the first 30 years of the Spanish conquests, from Columbus's first voyage and return in 1492-93 to Magellan's circumnavigation of the globe in 1519-22. It is an old-fashioned, almost self-indulgent narrative, and thus rich in its descriptions of characters, events and landscapes (it is also admirably illustrated). As a contrast, one might look at Henry Kamen's recent book, ''Empire: How Spain Became a World Power, 1492-1763,'' also a splendid work, but one more discursive intellectually, if much tighter in its analysis. Thomas prefers atmospherics and, in his case, it works.

His book begins by siting this tale of conquests in the context of the very recent ''reconquest'' of Spain from the Muslims, along with the unification of the monarchies of Ferdinand and Isabella -- and for very good reason. For to the poor but intensely Catholic noblemen and gentry of Castile and Aragon -- who had fought so fiercely to drive the Moors out of Granada and then pursued them into North Africa; who were locked in lengthy conflict against the Ottomans in the Mediterranean; and who had overwhelmed the Canary Islands -- it was only one step more to venture farther afield in pursuit of glory, gold and entry into heaven.

The people who carried out this mission were larger-than-life figures, many of them scoundrels, many of them ruthless, most of them obsessed. The greatest strength of Thomas's book is to bring so many of them to life -- so much so that one fears several swashbuckling Hollywood movies will emerge from this volume. The 10 chapters on the incredible story of Christopher Columbus cover not only his years of petitioning the monarchs and grandees of Europe to finance a venture to the West, and not only his extraordinary first exploration, but also his later voyages, including his epic fourth voyage in 1502-4, which was so full of setback and adventure. Thomas admirably recovers from history the character of Queen Isabella, surely one of the most inventive and decisive female monarchs of that era, along with Elizabeth I of England. There is the tale of Cortes's bold conquest of Mexico, told with a grand admixture of details on the religiosity of Cortes's men, their advantage in horses and fine swords, and the crucial support of the local tribespeople who hated Montezuma's blood-bath regime. Most empires rely heavily upon collaborators, and Spain's was no different.

Readers will learn as well the remarkable story of the missionary Bartoleme de las Casas, perhaps the most experienced of the doughty souls described in this book. He sailed on at least two of Columbus's voyages and survived numerous dangers, but then returned to Spain in 1519 to argue before the new King Charles I (who was also Emperor Charles V) against the dreadful cruelties being inflicted upon the Indian tribes in the West. In its way Las Casas' work marked the beginning of the Franciscans' and Dominicans' campaign for native human rights, undertaken because, as children of God, indigenous peoples were also expected to learn about Christ.

Thomas has researched in all the available Spanish and Latin American archives. He seems to have read all the sources. The index is a masterpiece. The 85 pages of endnotes are studded with interesting comments. The bibliography is vast (though I was a little disappointed to see no reference to William Prescott's 1843 classic, ''The History of the Conquest of Mexico''). This book is more than mere summer reading, yet I imagine that many people will eagerly lug it off to their cottages and resorts.

''Rivers of Gold'' provokes one further thought. For the past few years, the United States has been attempting its own imperial or demi-imperial experiments in Iraq and Afghanistan. Five hundred years after Cortes, neo-conservative adventurers are leading us eastward and seeking to transform the Middle East. But perhaps they should pause, at least long enough to read Thomas's book. It brings much evidence of imperial arrogance and torture, yet it also contains compelling details of how to treat a conquered nation with compassion. This is worth some reflection.

Many years ago Barbara Tuchman wrote a book, ''A Distant Mirror,'' about how the turbulences, extremism and brutalities of the Middle Ages were being echoed in our own time. Her book or, rather, her message was generally dismissed by reviewers, especially those who were academic historians. We might wish to treat ''Rivers of Gold'' more carefully. It stands on its own firm historical ground as a grand and sweeping account of the world's transformation half a millennium ago. But to those who enjoy analogies, it can equally serve as a memorial about empire and about imperial ambition.