They carried away precious Ming dynasty ceramics, rare books and scholarly works produced by Tang dynasty philosophers, 7th century Buddhist ceremonial objects, and countless other national antiquities. Before the end of the Japanese occupation in 1945, centuries-old museums in Beijing, Chongqing and the then-capital of Nanjing had been emptied and burned, and their contents spirited out of the country.
More than a half-century later, many of these national treasures have yet to be returned. Instead, they've surfaced in some of the world's most distinguished museums, such as the Palais du Louvre in Paris, the Metropolitan Museum in New York, the Prado Museum in Madrid, and the British Museum in London, while thousands more have gone to private collectors, sold in high-profile auctions or black-market deals.
Among the priceless treasures lost during World War II was a fossilized skull of Peking Man, one of the first examples of early man, excavated between 1929 and 1936 and about a half-million years old. Some believe it was pilfered by the Japanese from Nanjing in 1937, while others say it was stolen by U.S. forces, who were asked by Chinese museum curators to transfer it out of the country for protection.
According to Chinese art historians, more than a million artifacts, dating from pre-history to the end of the Qing dynasty in 1911, are currently scattered across more than 200 museums in 47 different countries.
Now, some of China's most eminent art historians and archeologists are planning to launch a campaign, on par, some say, with efforts under way in Europe to retrieve works of art plundered by the Nazis during World War II, to demand that the world's museums return thousands of national antiquities.
"There's absolutely no question that these national treasures were stolen from the Chinese people," Li Xueqin, director of the Center on Ancient Culture at the Chinese Academy of Sciences in Beijing, told United Press International. "We will fight for the return of our cultural relics, they rightfully belong to us."
China has joined other countries -- Turkey, Cambodia, Nigeria, Mali, Bangladesh and Greece -- that are planning to file a suit against museums from New York to St. Petersburg, Russia, to reclaim plundered antiquities. A complete list of the national treasures in overseas museums is being compiled by Chinese researchers and will soon be presented to the museums in a formal demand that they be returned to China, Li said.
The plaintiffs face a difficult task. In December, the curators of 18 prestigious museums in the United States and Europe issued a joint statement refusing requests by several nations that they return artifacts, including those that had been obtained illegally, or looted during times of conflict and Western imperial conquest.
"The objects and monumental works that were installed decades and even centuries ago in museums throughout Europe and the United States were acquired under conditions that are not compatible with current ones," the joint statement said. It argues that the value of the archaeological, artistic and ethnic objects in promoting culture outweighs the desire by individual nations or racial groups for their return.
James Wood, director of the Art Institute of Chicago, wrote in the letter that collections "have become part of the museums that have cared for them," and "part of the heritage of the nations which house them." He used the term "universal museums" to describe how the signatories, by displaying and protecting works of art from different peoples and cultures, offer a shared representation of the world's artistic undertakings.
Critics say the statement reflects curators' concerns about losing some of their most prized objects, many of which were acquired during the heyday of Western imperial conquest, between the 18th and 19th centuries. The statement presages the difficulties that many countries will face in their efforts to persuade European and American institutions to return artifacts from their collections.
One such example is the Elgin Marbles, known in Greece as the Parthenon sculptures, which date from between 447 and 432 B.C. and depict a formal religious ceremony of ancient Athens -- the Panathenaea procession. In 1799, the British ambassador to the Ottoman Empire, Lord Elgin, removed the sculptures and took them to London. Since then, calls for their return to Athens have been largely ignored, despite the launch of a major campaign to pressure the British government.
The museums argue the antiquities and artwork of other nations have been better cared for in foreign institutions, safeguarded against the threats of war, pillaging and social upheaval.
In the case of China, some art historians say, this may be a compelling argument.
"Many of China's finest works of art were actually rescued from decimation during the Second World War and afterward because they were transferred to overseas museums," said Michael Rhodes, a researcher in Chinese art history at Boston University's College of Art. "Whether they had been obtained illegally is a subject of some speculation, but the fact still remains that the items were protected by these institutions."
During the Cultural Revolution, from 1966 to 1976, when Maoist Red Guards went on ideological rampages throughout the country smashing the "Four Olds" -- old ideas, habits, customs and culture -- an uncountable number of China's major antiquities and dynasty-age treasures were burned or destroyed.
More recently, the theft and trade in Chinese archeological finds and artistic treasures has increased, with top customs officials reporting a dramatic rise in attempts to smuggle valuable artifacts out of the country.
But China's museum curators, like Wang Qingzheng, deputy director of the Shanghai Museum, say times have changed. They argue that newly built institutions in most Chinese cities are able to care for the collections of archeological, cultural and artistic treasures as well as, if not better than, their counterparts in the West.
"These arguments are a reflection of the patrimonial attitude of many Western countries, the belief that we're unable to care for our own cultural heritage," he said. "To them we say, 'thank you for taking care of theses items, but now it is time to return them to their rightful owners.' We hope they will respect that."
Amid mounting opposition by the museums to return plundered art works, China has adopted a new tactic in efforts to reclaim its past: museums in Shanghai and Beijing have been quietly buying back the nation's lost treasures, often spending exorbitant sums of money at auctions in Hong Kong, the United States and Europe.
When British and French troops attacked the Qing dynasty in the mid-1800s, in an attempt to force the Chinese government to accept the opium trade, they burned the emperor's Summer Palace in Beijing, looting hundreds of art works which have since exchanged hands in a number of countries over a span of more than 140 years.
In 2000, London-based auction houses Christie's and Sotheby's announced plans to auction several of the antiques that had been pilfered from the Summer Palace. The objects included three bronze sculptures -- the heads of an ox, a monkey and a tiger -- that once adorned a fountain clock representing the animals of the Chinese zodiac in the Summer Palace, commissioned by the Qing dynasty during the 17th century.
An official of China's State Bureau of Cultural Relics, in a letter to Christie's and Sotheby's at the time, challenged the legality of the auction, arguing that treasures should be returned to China under principles established in 1995 by the U.N. Educational, Scientific and Cultural Organization.
Despite China's protests, the auction houses went forward with the bidding. China Poly Group, a state-owned corporation once operated by the People's Liberation Army, stepped in at the last minute to buy the antiques, including a one-of-a-kind 18th century Qing dynasty vase, for an estimated $4 million.
Still, a major problem for Chinese researchers tracing the flow of art works to other countries has been a lack of documentation. Many imperial-era records were lost to the war, burned or destroyed during bombing raids by the Japanese, and information that does exist has often been gleaned through secondary sources.
Ma Baoping, deputy director of China's Lost Cultural Relics Recovery Program, the government agency responsible for compiling a list of stolen treasures, said the research and detailed documentation of the country's lost artifacts has been painstaking, and laments that many treasures may never be recovered.
"It might take the efforts of several generations to accomplish the gigantic task of getting justice for our lost relics. But we need to start doing it now," he said. "Cultural relics are a testimony of our nation's history and represent the embodiment of a country's ancient civilization, connecting the present with the past."