Was it neccessary for the US to enter the Korean War?
It is very difficult to answer this question as there is no way to know for certain what would have happened if the US had not entered the war. It certainly would have seemed necessary at the time.
US entry into the war would have seemed necessary because it seemed necessary to contain communism. Americans firmly believed that the communists were going to try to take over the world. Letting them take Korea without a fight would have encouraged them to take even more.
US entry into the war would have seemed even more necessary given what had happened in recent years. In 1949, two major things had happened. First, the Soviet Union had built (having been helped greatly by spying on the US) its own atomic bomb. Second, China’s civil war had been won by the communists. These two events made it seem as if communism was growing rapidly and needed to be stopped. People would have remembered how appeasing Hitler had not worked and they would have been eager to avoid the same mistake with the communists.
We cannot know what would have happened if the US had not intervened. Would the communists have taken Japan and the Philippines? That would have been very bad for US power. But we cannot know. All we can safely say is that it clearly would have seemed necessary at the time.