In Korean war, when USA and South Korea almost finish North Korea, China joined the war (China is going to support North Korea). USA don't want the communist to push back, so they want use nuke, but Soviet Union (USA's enemy) has nuke too, if USA nuke North Korea and China, Soviet Union will nuke USA, USA don't want to destroy the world, so they didn't use nuke.
In Korean war, when USA and South Korea almost finish North Korea, China joined the war (China is going to support North Korea). USA don't want the communist to push back, so they want use nuke, but Soviet Union (USA's enemy) has nuke too, if USA nuke North Korea and China, Soviet Union will nuke USA, USA don't want to destroy the world, so they didn't use nuke.