Germany and Japan favored military solutions more than the United States did. I think the militaristic nations felt they had more to prove on the world stage. For Germans, rearmament became a point of national pride. For the Japanese, imperialism played a similar role. By contrast, Americans entered World War I reluctantly, and they remained reluctant to enter World War II until the bombing of Pearl Harbor left them with few alternatives.
The correct answer is D.
The poem discusses thirteen ways of looking at a bird. Each way is different from the others. In this way, Stevens is illustrating how perspective varies from person to person and from moment to moment.
Even the same person can view a bird differently -- how he or she sees it on Monday may not be the same as how he or she sees it on Friday. Furthermore, how one person views it is different from how someone else sees it. Perspective is varied and numerous.