Past grievances born of even further past grievances born of a past scarcity in resources drive us to animosity.
Imperialism driven by a desire to extract resources from abroad in order to appease the people of the motherland.
Irreconcilable differences in culture born of separation and local conditions.
But most important today is the perceived necessity to defend oneself. One state cannot know the intentions of another state. What if they are building weapons to invade us? We MUST have our own weapons or we will be invaded. They are building weapons! If we do not do the same, we will be destroyed!
It's a
prisoner's dilemma with cataclysmic consequences. No state wants to simply allow themselves to be destroyed in order to allow the other to thrive on their corpse.
Governments and leaders with an incentive to increase morale and the willingness of their people to fight, to defend themselves, glorify war through propaganda and storytelling.
A grand ideal of unifying the world such that no one need fight again - to achieve peace through war.
The invention of tools to hunt prey for food.
The natural conclusion that those same tools work against fellow men.
The realization that taking from more prosperous men is often easier than hunting ferocious wild beasts.