Why one nation?
Humans are social beings who tend to live in big, complex, cooperative and competitive communities beginning in forming families and ending with forming nations. According to others, these communities can be classified according to their income, fortune, power, fame, and other factors.
A man is the only being who has a very developed brain that can think and use a language and speak and present solutions for the problems he faces. He is the only being who can use his hands skillfully to make mirco and macro tools that he needs in his life.