Communication, Transportationg of Messages, in Trade, and of Arms
A 1mountain
or ocean is a barrier to communication.
Distance, storms and dark had long separated communicants.
Wars have been fought for natural resources, too, maybe even 2the oldest recorded war.
But to turn the question around, why don't people on the _same_side_ of a border, or share a natural resource, attack each other?
Why wars? An ineffective form of conflict resolution.
Geography kept the Mayans and Egyptians from ever considering war with each other.
