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.