From the "letter of transmittal" attached to a December US Congressional report, "China's Impact on Korean Peninsular Unification and Questions for the Senate" (pdf):
When we consider the possible unification of the Korean Peninsula at some time in the future, the German model of unification often comes to mind. The purpose of the attached report is to alert Members that another outcome is possible. China’s historical claims to territory within the borders of the Korean Peninsula and the expanding investment by China within North Korea point to a situation where China may attempt to manage, if not oppose, the process of Korean Peninsula unification. The attached report includes extensive information regarding China’s trade and economic interaction with North Korea and the growing investment by Chinese companies inside North Korea.
For historical perspective, Senate Foreign Relations Committee staff obtained information about Chinese claims that parts of the Korean Peninsula were historically part of China, and South Korean assessments about those claims. The Congressional Research Service (CRS) was asked to write about how China presents its historic claims to the Korean Peninsula so that Members would be alerted to this situation. The Northeast Asia History Foundation in Seoul greatly assisted by providing South Korea’s view of China’s historic claims. Neither the committee nor I take any position on the disputes over the history of the Korean Peninsula discussed in this report.
Important questions are raised toward the beginning of the attached report for the Senate’s consideration regarding prospects for unification, the significance to the United States and our overall Korea policy.
The Taiwan-based China Times reports:
From a historical point of view, the Korean peninsula was always considered a buffer zone for China against foreign invasion and thus Beijing would be loath to welcome a reunification under a pro-American Seoul government.
Furthermore, it is not in China's national interests to face a strong and unified Korea on its northeastern border, the report said. Territorial disputes between Beijing, Pyongyang and Seoul occur frequently over the Chinese-Korean border. "China may try to impede the reunification of the two Koreas, which have been divided for more than 60 years, or seek to play a major role in a reunified Korea," said the report. "Disputes about the Korea-China borderline are historic and endless."
To maintain regional stability and prevent the collapse of the Stalinist regime in the North, Chinese officials told the Senate Foreign Relations Committee that China reserves the right to place PLA troops across the border inside North Korea, which would also be a means to prevent floods of refugees crossing the border into China. "These plans have been described not as an invasion, but as a preemptive move that will be taken in consultation with North Korean authorities," the official said.
That seems all too horribly likely. If - when - the North Korean regime shows signs of imploding, Chinese troops will cross the border, "not as an invasion", but invited in by their fraternal North Korean comrades to stabilise the situation. Then, like Tibet, justifications will be found as to why North Korea was always, historically, part of China...