Why did Mao Zedung get involved in the Korea War?

Mao Zedong, was the top leader of the Communist Party of China and the People's Republic of China. Under his leadership, the Communist Party seized power in mainland China in 1949, and it involved China in the Korean War because the People's Republic of China had warned that it would react if UN forces surpassed the The border on the Amnok River (and surpassed them), but before attacking Mao I hope Soviet aid and saw the intervention as essentially defensive.