You don't need me to educate me on Vietnam history, I am well aware of their historical interaction of Vietnam with China, hating and fearing China is in their genes. However on the other hand do you know the fear Vietnam have for China is the same fear that many Southeast Asia nation have towards Vietnam? Nan-yu (Vietnam) for over 1000 years was only around Hanoi, its only in recent century they expend south which give Vietnam the narrow shape they have now, and who do you think they took this land from? And the nation they took it from, what are their feelings towards Vietnam... hint Cambodia HATE Vietnam, that is also why Cambodia also loves China. Same thing goes for Laos and Thailand.
However my original point still stand with Korea, Korea might hate/fear China less than Vietnam, but Korea in its entire history was even more dominated by China than China dominated Vietnam (especially recent history). Therefore Korea share the same anxiety that Vietnam have over China. And I pointed in my previous reply, South Korean nationalism today is already making revisionist demand on China base on its distorted history. What do you think a unified Korea will do?
When Vietnam was not unified, they didn't know any similar ambition and trouble towards China of the current South Korea is showing towards China today. To think that China can manage a unified Korea is laughably as to think China can mange a unified Vietnam.
But in a alternative history, if Vietnam today is still divide, do you think they would pool their full attention toward being hostile towards? Against each other? Therefore Korea unification is not good for China.
First off, I was not intended to "educate" anyone, but to present the historical events as a reference. My words was "let US revisit..."
Secondly, I think dragging in more countries' history and feelings towards one another (one against Vietnam for example) will get the discussion even more messy. I disagreed your parallels of Korean matters with Vietnam in my previous post, and I still suggest now not to bring in parallels which may not be relevant.
Thirdly, I believe there is no meaning or usefulness in "what-if" in history.
Lastly, for the sake of debate, I will to go along with your hypothesis of "what if Vietnam is still divided".
Are you suggesting that the only good thing for China is to have all her neighbors and even better countries all over the world being divided and in chaos. Only that will fit China's interest? What kind of interest is that then? I love China being the place of my ancestors, BUT I do not wish my neighbors harming their brothers and sisters just so they don't have time to POSSIBLY think of hurt me. When people do not point me with a knife, I give them the chance of being a friendly neighbor instead of assuming them to be criminals. And I do keep a big stick just in case.
Regarding the hatred (against each other) that you mentioned, they are true. But I have also heard lots of very nasty words of Chinese in the internet against other countries regardless right or wrong of the country in the specific case or subject or event. We should not let this kind of sentiment to dominate the decision and future of countries including China even the noise is there.
Regardless who did what to whom in the past, human must learn to let go and work towards a chance to be good, not act in ill suspicion against each other because of dark past. Otherwise, the bad will be a reality, self proven prophecy.
I know you have heard this two sayings of the Saint Confucius, (not to educate you), let us share it with everyone.
己所不欲勿施于人, One do not treat others in the way one do not want to be treated
China do not want to be divided, so China should not do anything to stop Koreans.
君子成人之美, True gentleman help or willing to see other person being fortunate/happy
If Koreans wanted unification, China should be fine to see it happen.
Yes, I know realpolitik and dirty tricks, but I also know that they only work in short term, not long term. China out survived all other empires to this day is NOT because she outsmarted all others with more dirty tricks, BUT because she had the Saint who guided China for 3000 years with many wisdoms including the two sayings above.