Contradictory historical claims by China and South Korea over an ancient kingdom that ceased to exist 1,300 years ago has led to an unlikely alliance between the two Koreas, with important implications for a future reunified Korea. The warrior kingdom of Koguryo, which for almost a century straddled most of the Korean peninsula and a considerable part of what today is northeastern China, was dragged into the public spotlight last year by a Beijing-backed study that concluded it was historically an integral part of China. The claim has been fiercely rejected by South Korea and the country's academics who consider Koguryo (BC 37-AD 668) to be one of the founding kingdoms of the country and the basis for the word 'Korea'. 'It is an indisputable historical fact that Koguryo is the root of the Korean nation and an inseparable part of our history,' South Korean Foreign Minister Ban Ki-moon said. 'We will sternly and confidently deal with any claims or arguments harming the legitimacy of our rights,' he said after the Chinese Institute of Social Science released the report as part of its Northeast Asia project. The issue has also struck a raw nerve with North Korea, which is lobbying to have Koguryo temples placed on Unesco's world cultural heritage list of historical sites. It has been blocked by Beijing, which is presenting Koguryo tombs in northeast China as substitutes. In recent months, the North Korean media has published several broadsides against claims by Chinese academics that Koguryo was a vassal state that maintained a tributary relationship with China. Koguryo fended off successive waves of invading Chinese armies of the Sui and Tang dynasties to defend its sovereignty, noted North Korea's official newspaper Minju Joson. South Korea has also responded by setting up a research institute with the task of supporting the territorial and cultural sovereignty of the kingdom. The historical dispute goes beyond accusations of historical revisionism and highlights long-term, potentially competing geopolitical ambitions for the area. 'China's Northeast Asian project is not just about Koguryo but aims at asserting its historical claims to Manchuria and even part of the Korean peninsula in case the region turns unstable,' said one South Korean historian. China's worry is that the possible collapse of an impoverished North Korea would release a flood of refugees and instability in the area, forcing it to establish a new frontier against a US-backed, unified Korea.