Rarely is religion a primary reason for war. It is usually because some neighboring country has something that another wants; land, resources, food, more beautiful women, whatever.
Religion is one way to make people who usually would not think of murdering their neigobors follow the lead of those planning the war by de-humanizeing those being planned for the slaughter. Convince them that if the neighbor is not a believer that they are, evil, sinners, unworthy of life, have something that 'god' wants you to have because you are faithful, yada, yada, yada.
Religion has been a tool of war, but not usually the reason one has started.
Jean