China restricts academic access to historical archives
If Chinese officials provide any explanation for recent restrictions, it is usually that the materials are being digitised.
But some historians believe that restrictions, which have not been officially declared, are related to heightened sensitivity over historical claims in the East China Sea, as relations with Japan became strained.
Other historians suggest that blanket bans on some archives may be related to the current government's desire to control the historical image of the Communist Party and protect it from critical analysis and attacks by scholars.
"Many people have noted that the Chinese have steadily been removing files and documents from their archives," said Ivy Maria Lim, a historian and assistant professor at Singapore's National Institute of Education.
"Researchers are in flux because they can't get access to information."
The Nanjing archives
Documents have disappeared from the official archives in Nanjing, China's former 'southern' capital. "They have been removed on the grounds that they are digitising them. [The authorities] said the files will not be coming back for the next decade or something like that," said Lim, co-editor of the book Controversial History Education in Asian Contexts.
While many historians welcome digitisation, "the difficulty is that you cannot know what hasn't been digitised and you are more at the mercy of the archivists' own selection in terms of what has been digitised," said Rana Mitter, director of the China Centre at Oxford University and a professor of Chinese history.
The Nanjing archives contain important documents on the city during China's Republican era after 1912, the Japanese occupation in the 1930s and 1940s, and the Communist period after 1949.
Japanese troops occupied the city in 1937, leading to a major massacre known as the 'Rape of Nanjing'. However some ultra-right Japanese politicians have inflamed already sour relations with China by denying the massacre took place or by downplaying it.
Bilateral ties between the two countries have been severely strained by disputes over the Diaoyu islands in the East China Sea, known in Japan as the Senkaku islands, and the visit by Japanese Prime Minister Shinzo Abe last December to the Yasukuni Shrine that honours the country's war dead, including 14 leading war criminals.
Although the Nanjing archives remain open, historians report that vast sections are now out of bounds.
"You don't normally digitise the entire archive, you digitise in parts and keep the rest open. It's pretty scary in the sense that there is no announcement made about what's the basis" of the restrictions, said Dr Barak Kushner, a senior lecturer in Asian and Middle Eastern Studies at Cambridge University. His speciality is Japanese history.
"Until recently the Nanjing No 2 Archives were not releasing documents but were publishing stuff they selected from the archives. It has been a concerted effort of the government to really push that sort of release over the last few years. But now the archive is completely closed for, quote, digitisation, unquote," said Kushner.
Foreign ministry archives
The foreign ministry has also said it is "in the process of upgrading its computer system".
"It has become much harder to access certain archives that were formerly in the mainstream of what you could access easily," said Mitter.
The ministry's archives on China's dealings with other countries have been all but closed. "In the last year it has proved almost impossible to get hold of material from there," he told University World News.
Many other researchers, who preferred to remain anonymous to preserve their good relations with the Chinese authorities, reported similar restrictions where none previously existed, but believed scholars from the most prestigious universities were still being allowed in.
Few believe digitisation is the main reason for limiting access to foreign academics, and in many cases, to historians at Chinese universities.
Kushner said China may have become nervous after an article in a Japanese newspaper in December 2012 quoted a 1950s document from China's foreign ministry archives which apparently used the Japanese name Senkaku for the Diaoyu Islands.
The article suggested that archive access had been restricted because the incident appeared to compromise China's historical claims.
According to Japan's Jiji Press, access to China's foreign ministry archives had been 'strictly limited' since January 2013.
Some provincial archives have also been closed to foreign researchers. Even archives such as those in Shanghai, which have had an open door policy in the past, have tightened their rules, insisting that academics produce a letter from a local Chinese organisation stating specific areas of research, before being admitted to the archives.
"It has become extremely difficult in the last year now in China to do archival research," Kushner said. "There's no access to Communist Party archives either. It's based on your internal connections, if you have any, which means that only some Chinese scholars can get access."
Chinese scholars have been affected. Last year Zhu Jianrong, a Chinese professor of international relations and history based at Gakuen University in Tokyo, was held for six months in China and released on 17 January.
Zhu declined to go into details of his detention and interrogation when he returned to Tokyo on 28 February. He said he was questioned about materials he was gathering as a researcher of modern history and his role in promoting Japan-China relations.
"[Chinese] investigations found nothing inappropriate," he told Japanese media.
Where archives are still open, "the distinction seems to be between archives, or subjects that may be more closely linked to the Communist Party itself. A lot of archives to do with the World War II period [when there was] a nationalist rather than a Communist government, continue to be a fairly accessible," said Mitter.
This is particularly the case with documents that have little direct bearing on China's contemporary policies.
Kushner concurs. "Japan is just used as a lever. There is a linkage with [current relations with] Japan but that is not the cause; it is not all of what we are seeing. There really is an internal shift within China."
The government under president Xi Jinping "has begun a large-scale campaign to whitewash history" according to Willy Lam, adjunct professor at the Centre for China Studies at the Chinese University of Hong Kong.
"Much more so than previous leaders, Xi has refused to let party members or ordinary intellectuals talk publicly about aberrations of the Communist Party, especially those committed by Chairman Mao."
But shutting off archives has much wider implications, pushing academics towards certain topics.
"It limits not just what foreign academics and researchers can work on but also what scholars in China can work on," said Kushner, adding those who do have access "are the anointed ones and they therefore control our understanding or limit our understanding [of history].
"It puts a chilling grip on what a scholar can research. The message is don't step out of line, don't work on certain things, don't go against the opinions that we are saying. It turns history into being a toady of the government."