Religion

One way of constructing a moral distinction between "us" (good) and "them" (evil) is religion.
Some forms of religion are non-exclusive; they accept any number of gods. These can hardly inspire wars.
The common mono-theistic religions are more dangerous.
Let us have a look at the 10 commandments:

1. I am the Lord thy God. Thou shalt have no other gods before me.
2. Thou shalt not make unto thee any graven image.
3. Thou shalt not take the name of the Lord thy God in vain.
4. Remember the sabbath day, to keep it holy.
5. Honour thy father and thy mother.
6. Thou shalt not murder.
7. Thou shalt not commit adultery.
8. Thou shalt not steal.
9. Thou shalt not bear false witness against thy neighbour.
10. Thou shalt not covet thy neighbor's house, neighbor's wife, neighbor's servants, animals, or anything else.

1. Ensures that non-believers can not live morally.
2. May be contrary to what those Others in the neighbouring countries did at the time of writing, serving as another moral distinction from Them.
3. Ensure the word Jahwe does not change its meaning (increases meme stability).
4. Another law to distance yourself from those Others, and surely popular.
5. Try to ensure that the religion is transmitted to the next generation.
6. A common moral law already manifest in our genes since long before we became humans. Viewed in the context of the Old Testament, this tabu is restricted to the own tribe.
6.-10. Meant to reduce inner-tribe conflict.
8.,10. Establish property.

Memes evolve to maximize their stablity and transmission rate.
Religious writings claim to be the words of the creator of the universe, which gives them utmost importance. This ensures they are not easily forgotten, and can influence the behaviour of their host.
Religion is usually transmitted from parents to children (if both parents share a common god).
Another road of infection is attempted by commands to spread the word. In practice, this is rarely successful, if the neighbor is already infected with some other jealous god. Therefore, spreading a jealous god meme to a new area is generally only possible by warfare.

In theory, if law 6 is interpreted to apply to all humans, it should make warfare impossible. But in practice, wars have happened everywhere. Obviously soldiers are exempted from law 6. They have a licence to kill.