In the aftermath of WWII, several countries were divided: Austria, China, Germany, Japan, Korea, and Vietnam.
Austria was reunited by international agreement in 1955, probably because the Soviet Union preferred a neutral, united Austria to the possibility that the American, British, and French zones would unite with West Germany.
China remains divided; the Chinese government, while insisting on one China, has chosen to bide its time to avoid a potentially disastrous conflict.
Germany was reunited after the collapse of the Soviet Union. However, large parts of what once was Germany are now incorporated into other countries.
Japan lost Sakhalin Island to Russia and no longer claims it. Japan does claim the South Kuril Islands also claimed by Russia. The status of Okinawa and the Ryukyu Islands is too complex to discuss here.
Korea remains divided, but like China, both Korean states support unification.
Vietnam was reunited as a result of North Vietnamese success in war, the only case of reunification by war.
So why is Korea still divided? The Korean War, which can be seen as a failed attempt at unification, is a likely answer. Fear and resentment due to the war, the vastly different economies, and the influence of rival powers, have likely made unification unlikely in the foreseeable future.
One can imagine what North Korea might hope to win from prospective negotiations:
- A fair and equal peace treaty with the United States and other countries.
- Recognition of their status as a nuclear power.
- Relief from economic sanctions.
- End to US involvement in military exercises.
- End to US arms sales to South Korea.
The United States and South Korea might hope for some reduction in fear of North Korean militarism. What form this might take, what North Korea might agree to give to achieve its goals, is harder to imagine.
New NY 23rd 
Kim has already won the negotiations with Trump. The fact that Trump has given this brutal dictator equal status with the US is a major victory for Kim. Kim continues to oppress people in his own country, violating basic human rights. For this reason former POTUSes have not agreed to give him high status as a “leader”. he is not a leader, he is a brutal dictator in the same vein as Saddam and Assad.
This is not to say the the US should not be exploring diplomacy, but not at the highest levels. First, NK should be required to make improvements in human rights and agree to freeze new nuke research and development. Only when those assurances are reached at lower levels of diplomacy should the POTUS even consider meeting on an equal footing with Kim,
I am still doubtful that the meeting will ever take place. I think that the Trump warmongers are going to use this as a pretext to attack NK, especially as the Mueller investigation gets even closer to Trump. They will say: We tried, but Kim would not budge. It is a pretext for war.
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I do think that Kim sees an opportunity to exploit Trump’s naivety, and may well succeed in “eating his lunch.” I also fear that if rebuffed, Trump may react with anger rather than reason.
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An interesting and relevant article:
https://www.38north.org/2018/04/sblank041818/
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In the First Gulf War, having driven the Iraqi army out of Kuwait, the American army stopped. In the Korean War, after driving the Northern army out of the South, the Americans kept going. Why it was wrong for the North to attempt to reunify Korea by force and ok for the Americans to attempt the same thing is a puzzle.
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It was not right in Korea. While the initial war was fought due to northern aggression, the attempt to take over the north was a later decision by Truman based on the flawed “domino theory”. Should not have happened. Should have established the border and left it at that.
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A decision by President Truman? Officially, it should have been a decision of the UN. In practice, General MacArthur, who may have sought a wider war with China, may have been out of the control of both President Truman and the UN. Truman seems to have given MacArthur a free hand until MacArthur was suggested as a candidate for President–Truman well understood that threat.
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