From the beginning of humans, we have created boundaries between ourselves by lots of conflict over the century. Our greediness and eagerness for more power make us intelligent animals. Humans are capable of their own destruction. But what if those never happen? what will be the scenario of the current world if there is only one nation and one currency and humans invest their full strength for human resource development and the development of science and technology? What do you think is that even possible in the future and what will be the benefits and inconvenience of it?
Can you imagine a society without hierarchy? Can you imagine anarchy over governments? One could make up things in mind but it's impossible to overturn the reality.
One currency is possible and already exists. Everyone in every place will happily accept gold, diamond and other precious stones.
But it's impossible to form one nation because humans are inherently social animals, social grouping species. Observe, family members that were raised in the same area, are different from each other, neighborhoods in each area, are different from each other. Different cities have different cultures and attitudes. This applies to different countries and even different continents. Even the city Barcelona wants to get out of Spain, even South Korea and North Korea are hugely different.
See how Asians differ from Europeans, South Americans from North Americans, etc.
After this, do you really think that we can form one nation? We can only do that in a limited area, that's our maximum. People will always form social bonds, people will always confront each other, that's just our nature, our hormones. The reason why men are aggressive compared to women is that men have high testosterone levels while women have high estrogen and oxytocin, both of which make them emotional and empathic. Even our brain hormones differ, men have higher dopamine levels than women, which leads men to make more risky tasks, chase success, motivation, etc. This is a very complex task, it will take a lot of time for me to explain clearly and I assume it will probably be boring for the audience. But I hope you get the point!