Beijing ? Forgive Gavin Menzies for feeling a little defensive.

His book, “1421: The Year China Discovered America,” may be selling briskly in the United States, but his extraordinary theory that Chinese explorers reached the New World decades before Christopher Columbus is proving a tougher sell to academics — even here in China.

“Nonsense,” declares China’s Zheng He Assn., which celebrates the exploits of Zheng He, the very explorer Menzies says directed ships around the globe a century before Ferdinand Magellan.

But Menzies isn’t fazed. “I don’t see how any fair-minded person who reads the evidence can come to any other conclusion other than the Chinese did get to America before Europeans,” he said in a telephone interview from New York, where he was promoting his book.

If only it were that simple.

China in the early 15th century was a great seafaring nation; no dispute there. Huge Chinese ships bearing silk, porcelain and other treasures made epic expeditions at the emperor’s behest. Commanded by the admiral Zheng He, the ships traveled from China down to Indonesia, west to India, and as far as East Africa.

But this is where Menzies departs from established history. He says he has found proof that the Chinese ships sailed on — around the Cape of Good Hope and all the way to the Americas, with some ships even crossing the Pacific back to China.

Menzies, a former submarine commander in Britain’s Royal Navy, insists not only that Chinese beat Columbus but that European explorers who reached the Americas did so with maps copied from the Chinese.

“All of the great European explorers set sail with maps showing their destinations,” Menzies said.

His book, published in the United States this month, entered The New York Times’ nonfiction best-seller list two weeks later at No. 8.

Menzies says he has received support for his work but concedes that some experts have expressed strong reservations. His critics argue that China’s huge wooden ships couldn’t have survived the rough Atlantic voyage. Some also say Chinese and European cartography at the time was so different that the maps couldn’t have been reconciled.

A replica of Christopher Columbus' ship Nina makes its way under the Golden Gate Bridge in San Francisco in this June 24, 2002, file photo. A new book published in the United States asserts that it was Chinese, not Europeans, who first reached America in the early 1400s.

Others call his book “rubbish from beginning to end,” Menzies acknowledges. That includes some in China, even though the book hasn’t been published here.

“It’s crazy talk,” said Wang Xiaofu, a history professor at Peking University. “We absolutely do not accept this theory.”

Many Chinese authors have presented similar theories over the years. Some even argue that Chinese settled the Americas 3,000 years ago, Wang said. But most tales mix fact and legend.

“In ancient times, there were a lot of fairy stories,” he said.

Menzies, who lives in London, sailed the routes of Columbus, Magellan and other European explorers when he was a naval officer. He writes that his knowledge of maps and using the stars for navigation led to his theory, and that his research took him to 120 countries and every major port of the late Middle Ages.

He is hardly the first to challenge the story of discovering America. There is evidence of Viking settlements in North America 500 years before Columbus. And humans are believed to have walked from Asia across the Bering Strait when it was covered with ice to become American Indians.

Columbus’ achievement was not so much to discover America as to open it to European conquest and competition to settle the New World.

And China, by the middle of the 15th century, had isolated itself. Its treasure-bearing ships were summoned back, and the emperor forbade overseas travel. China had halted all exploration, leaving the world to Europe.