Japan is considered to be a victory in the Cold War because it became a strong ally of the United States in political terms and because its economy became a capitalist powerhouse.
During the Cold War, the US felt that it was important to keep communism from spreading. There was a great deal of communism near to Japan (China, North Korea) and there were those in Japan who favored communism. Therefore, it was a victory for the US when Japan remained firmly in the Western, democratic, and capitalist camp. Instead of becoming communist, Japan became an outpost from which capitalism and democracy could spread.