Internationally: today’s external patron (the United States) of the free Korean half is weakening, while the external patron (China) of the communist half is strengthening. The opposite was true of the United States and West Germany, and the Soviet Union and East Germany, in 1989. Today’s northern patron (China) is trying to push further into the Asian continent, while yesterday’s eastern patron (the Soviet Union) was looking for an exit from central Europe. Chinese peninsular intervention is therefore easier, while U.S. support for South Korea’s unification terms will be more difficult.
Here is part of the abstract from his 2011 paper: