Why does religion cause war?

Answer:

It doesn't, particularly, but those who want war sometimes use it as an excuse to attract support from those with no material stake in a conflict.
James Harrington, observed that "if war were for land, where one man had 1000 acres and another has one acre, the man with one acre would not venture so far as the one that had 1000, but religion is equal to both. Then all men would say is fight for land.  

A: 2

Religion was not the reason for the wars. Greedy opportunistic people who crave absolute power and authority are the cause of religious wars.
Religion gave politicians and the rich individuals the tools to control their workers and servants.
Crafty individuals cleverly exploited the ultimate human fear of death. Their objective was achieved by creating an abstract concept of Heaven and Hell so that they could induce their workers to maintain loyalty and obedience through the fear of sin and death.
They used natural acts of nature to authenticate their ingenuous fabrication. They convinced the gullible workers that these natural disasters were demonstrations of God's wrath upon them for their various misgivings from time to time.
When a competing religion threatens the economic control of another then terrorism and persecution is used to destroy the competition. Religious wars have more to do with wealth, power and greed, than it has to do with righteousness and piety.

Ever hear about the crusades? Christian Europeans hated Muslim Arabs and to stop the spread of Islam they tried to destroy it