Nationalism

Nationalism serves the interests of the rich and powerful, not the people.  There is no real difference between a person based on the location of their birth, the differences relate to wealth and power.
 
Human history is really rather brief.  It wasn't all that long ago that the very idea of a nation simply didn't exist.  Going back to pre-history is of course fairly speculative, but we have examples from the historic period that confirm that we originally coalesced our groups by clans or extended families.

This is fairly obvious since the essential thing almost all humans share is being part of a family.  In fact in those times, not having a family to protect you most likely meant you died or ended up as a slave.

Of course over time the need to survive led to clans becoming tribes large enough to provide defense or in some cases expansion for their members.

Now tribes require leadership and a certain amount of loyalty.  It was loyalty that required you to consider people not directly related to you in a manner similar to the way you viewed your kin.  Of course one element that made that somewhat easier to do was that tribes tended to be self contained, where over time many members had connections through blood or marriage to most of the other members.

Now nationalism is generally viewed as an outgrowth of tribalism where the tribe becomes expanded and the loyalty for the tribe gets transferred. 

Nationalism because of its scope encompasses quite disparate groups, all of whom end up being loyal to some central idea.  At one time this was embodied in a king or prince who became the embodiment of the state.  However, whatever the symbol it represents something greater.  In fact its the very essence of this relationship that we acknowledge a greater good.

Now every member may have their own view of what the greater good is.  Its the loyalty that matters not the object to which the loyalty is given.  In fact nationalism prefers blind loyalty, my country right or wrong is preferred.

Maintaining such loyalty requires constant effort to convince the citizens that they are better off in this particular country.  In many cases the lives of a citizen in country x and country z are indistinguishable, but they are frequently told how much better off they are to be where they are.

It is also important to define a common enemy against whom the tyrant can offer protection. 

It is this us vs them that allows soldiers to sacrifice themselves for essentially strangers.  They serve the greater good, or at least they are convinced they do.

The horrible wars of the last century were in general caused by extreme nationalism and millions upon millions sacrificed their lives.  The actual differences between most of the countries at war were minor in that they shared common histories and religions as well as traditions.  Yet they vilified each other.

We are seeing a rise of extreme nationalism as some people feel that they are losing their national identity and view that as a negative thing.

It isn't, the ideal state of the world would be where we treat each other as brothers and sisters working for the common good.

The idea that we are playing a zero sum game is what allows them to continue this illusion, "they" are after what you have.  Yes, assuming you have more than them, they might want to have the same, but they want their own, not yours.

We ultimately need to be loyal to humanity and the earth as a whole, it is really a spaceship and we are all in it together.