BEIJING - In a glass case at Beijing's Imperial College, an 18th century book with a yellowed title page in bold, black characters is evidence - some Chinese say - that a swathe of modern-day Japan belongs to China.
The two Asian powers are already at loggerheads over a set of tiny uninhabited islets in the East China Sea, even stoking fears of armed conflict.
But the most aggressive Chinese nationalists - tacitly encouraged by authorities - say far more is open to claim, including the island of Okinawa, home to 1.3 million people and major US military bases.
The biggest of the Ryukyu Islands, which stretch for about 1,000 kilometres (620 miles) from Japan's mainland almost to Taiwan, Okinawa was the centre of the Ryukyuan kingdom, which pledged fealty to both Chinese emperors and Japanese feudal lords.
For hundreds of years it paid tribute to China's Ming and Qing dynasties, until it was absorbed by Japan in 1879.
The people of the Ryukyus are considered more closely related to Japan in ethnic and linguistic terms, than to China.
Some Chinese, however, see historical and cultural ties as a basis for sovereignty and dismiss Japan's possession of the islands as a legacy of its aggressive expansionism that ended in World War II defeat.
"This kind of thing proves Ryukyu is China's," said electrical engineer Zhu Shaobo, looking at a Qing dynasty volume from the 1760s about Ryukyuan students on display at the Imperial College, now a tourist site.
"Ryukyuan students studied hard and the cultural level of some was not inferior to Chinese students," explains an exhibit panel at the institution, which trained Imperial officials and some foreign students.
The belief that China has a legitimate claim to the Ryukyu Islands has existed among flag-wavers in China - and Taiwan - for years.