On eNotes, we ask that you submit one question at a time. Therefore, I will answer one of your questions. The first is easier to answer if you just look at a timeline of the Cold War, so I will answer the second.
It is, to some degree, a matter of personal opinion as to whether the Korean War was a success for the United States. I will present both sides of the issue so that you can see which argument makes more sense to you.
We could say that the Korean War was a success for the US because the US was able to get things back to how they had been before the war. The US was happy enough with the status quo before the war, so getting back to that status quo can be seen as a success. Before the war, North and South Korea were divided at the 38th Parallel. The Korean War started when North Korea, which was communist, invaded South Korea, which was not. The US (along with other nations) entered the war to repel the North’s invasion. During the war, the US pushed the North back and the war ended with the boundary being set at essentially the same place it had been before the war. Thus, the US was able to protect its South Korean allies and get their country back for them.
However, we can also say that the war was a failure. The US pushed the North out of South Korea relatively early in the war. In just a few months, US forces had gone so far into North Korea that they were nearing the Chinese border. At that point, China became worried and entered the war on the side of the North. The US was pushed back out of North Korea. The war continued for a couple more years with neither side being able to get anywhere. This led to an armistice. Looking at it this way, we can say that the war was a failure because the US was not able to hold all the territory it had taken. If it could have, it would have destroyed North Korea and would have been able to unify Korea as a single country allied to the US. This would have been better than having so many soldiers killed and wounded just to get things back to the status quo before the war.
Which of these arguments do you find more convincing?
That question is perplexing. The United States was successful in the Korean War in that South Korea remained after the war as it was before. But what is "successful"? Is it the millions of dollars spent on munitions and other war materiel though that itself provided employment for the manufacturers and the soldiers and support personnel. Any war is unproductive, there are victors and vanquished, and hostilities remain. For the United States to be involved in international conflicts such as in Korea and in today's theatres of conflict is in sharp contrast to its position of non-involvement or neutrality prior to World War 1.