It seems obvious to me - it's the combination of two ideas:
1. When competing for resources, killing your neighbour frees up resources, which you can take. Most species of animal and even plants do this to some extent.
2. By collaborating in a group, you can achieve more than individuals acting alone. This is the idea behind teams, companies, countries, etc.
1. When competing for resources, killing your neighbour frees up resources, which you can take. Most species of animal and even plants do this to some extent.
2. By collaborating in a group, you can achieve more than individuals acting alone. This is the idea behind teams, companies, countries, etc.
Combine the two ideas, and you get war.