For China:
1) extend its influence further to Pacific.
2) become dorminat country and power player in Asia
3) test long developed weaponary and troops
4) establish a colony or special economic zone in N. Korea to help China economy with its cheaper labor
5) get real and show deterrance to Taiwan and Japan
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For US:
1) a gaurantee of nuclear free Korea
2) elimination of one of 3 axies of evils
3) join-venture with China for economic development and security in Korea
4) long-term occupation of South Korea
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For S. Korea:
1) No threat as long as China stay in the North