The USA and USSR where enemies, both watching their side of the pond waiting to strike. Their opportunity came during the Korean war.
After WW2 Korea was split in 2 and in 1950 North Korea invaded south Korea leaving them with only a tiny part of the island.
While North Korea was communist, South Korea was capitalist.USA and South Korea where allies. The UN and USA sent troops to communist occupied Korea and together South Korea and America began taking Back the peninsula.
Now, the roles are reversed. South Korea and the U.S. have taken most of the peninsula leaving North with only a small portion of the Peninsula.
China saw North Korea struggling and sent help and chased South Korea back to the 38th Parallel.
Now that they are back at the 38th parallel there has been no game ending change to the Korean war ending it with a stale mate.