Is war necessary?

Can there be a world without war?
I think yes, it is possible for humans to exist without going to war, but it isn't possible for humans to exist without conflict. It is in our instincts, after all, and you can hardly deny those.
But I don't think it is exactly instinct for us to create such huge forces and throw them against each other. A world without war may be impossible to achieve at this point, but I think it once was possible. Let me explain why.
The way Earth is divided into countries is a huge factor in war. If we were in small groups or lived by families, without being American or Italian or English but being an [insert last name here]. There would still be fights, obviously--we probably can't live without those--but there would be far less reason for an all-out war. War comes when people have enough numbers on their side to sacrifice others for a cause. This is easy with such large countries. Fewer numbers means fewer people will die for the sake of their country or family.
I don't think that people will abandon their countries now. The world is too far developed for wars to not break out unless we as human beings make a huge change.