Why do some nations fare well economically, militarily, and otherwise and others not so well?
There are many perspectives and variables. In light of this, let me give you two ways to think about this.
First, generally speaking if a nation has a lot of power to begin with, such as people, natural resources, money, strategic location, and the like, then that nation is set to do well. Moreover, wealthy nations tend to get wealthier, as they have more resources at their disposal. We can see this in America. We are rich in resources, have a great population, and continue to dominate in the world scene.
Second, from a historical point of view, things are much more complicated. Why? All nations rise and fall. In the second century (AD) who would have thought that the Romans would be eclipsed one day? Likewise even one hundred years ago, who would have ever thought England would not be a superpower? The point is that there is time of decline for all nations.