This evidence is unlikely to make a public appearance any time soon.Meanwhile, the foreign ministry has all but closed its records as archivists hunt for other politically embarrassing papers.

The unfinished business of 1945 in east Asia is coming back to haunt the region. Since late last year, observers have become increasingly alarmed at the appearance of Chinese warships and Japanese fighter jets around the disputed islands.

In May, two Chinese researchers published an essay in the – the most official of outlets – that went further than any Chinese territorial claim in the region had done so far, arguing that Ryukyu, the archipelago off the southern coast of Japan that includes Okinawa, had been ceded to Tokyo’s control in 1895 at a time of Chinese weakness at the end of the first Sino-Japanese war, and should be returned to China.

Meanwhile, in Japan, the prime minister, Shinzo Abe, has been raising the temperature on Sino-Japanese relations.

In late July, in the lead-up to elections to the upper house of parliament, he declared that Japan should change the phrasing of its 1993 official apology, in which the Japanese confessed to inflicting “immeasurable pain and incurable physical and psychological wounds” on the “comfort women” from China, Korea and south-east Asia used as sex slaves by the military during Second World War.

He has also tried to hedge around Japan’s invasion of China in 1937, arguing that there is no clear definition of the word “invasion”.

Before the election, Abe made it clear that the islands are Japan’s “unique territory, historically and in terms of international law”.

Abe’s hardline stance helped his performance in the 21 July election for the upper house of the Diet; his Liberal Democratic Party won a landslide.

The foreign ministry archives in Beijing have been hard to gain access to this summer.

For some years, documents stored there have been the source of some of the most exciting new research about diplomatic relations in Mao’s China.

But recently the flow of papers has diminished to a trickle.

Rumours suggest that a researcher found a document from the Mao era that failed to back up Chinese claims to sovereignty over the barren islands in the East China Sea known as the Diaoyu to the Chinese and the Senkaku to the Japanese.