Four thousand six hundred years ago — there were a series of devastating droughts that occurred around the Mediterranean after the Ice Age ended. One brought down the Sumerian civilization and set people to migrating.
These migrations due to climate change are devastating, then as now. One of the more pleasant places to live during the Ice age was the Middle East, present day Egypt, Ethiopia, and across the northern tier of Africa. Arabia, today a desert land, at that time had abundant water in the area today known as the Persian Gulf and was dry land.
Once people start migrating due to climate changes, displacements lead to war.