Why did China get involved in the Korean War?
Both of them held different parts of Korea and had different policy, the PRC government helped North, and ROC helped South, and after 1950, Kim Il Jung tried to unify two parts of Korea into one, and the US supported democracy, so they helped the South, and China, Russia supported communist, so they helped North. Also, China worried about US would break into China if the US and South Korea were allowed to win the war.