South Korean Response
South Korea's response to North Korea attacking them was to attack or get help attacking them, in which they turned to the allies. When North Korea had attacked them to the brim of the south side, they had asked the allies for help, which concluded in them to push back and force North Korea to retreat, South Korean responses were of defense.
The role of South Korea's response was major, since it was the other part of Korea being separated. It was really important that South Korea had chose nationalism and to fight communism, or else more communism would have spread, which meant more achievements towards Soviet's goals of spreading Communism. If South Korea had chosen communism, then there wouldn't be the need for Korea's separation, and the Korean war would be completely different.