The United States established a permanent military presence in Asia.
The United States proved that communism was a threat around the world.
The United States helped ease tensions between China and the Soviet Union.
The United States failed in its first attempt to prevent the spread of communism.
I would go for A, because it is still there until today. It has not proved more than the Vietnam war. At that time,there was no conflict between China and USSR. And it did not fail.I dare to choose A.