Religion does not cause war. People use religion to manipulate people into hating one another and fighting one another and that causes war. They use religion because it always touches the emotional side of people's lives. They do not think logically before they act where religious or what they believe to be religious issues are involved.
A lot of people in power throughout history have used religion to manipulate masses of poorly educated and superstitious people in order for them to gain wealth and power. This is grossly unethical and as people have become better educated they have condemned war. However, we are entitled to go to war to defend ourselves if we are under attack from as enemy regardless if that enemy is using religion as an excuse for waging war or not. We don't have to believe in the religion to find ourselves under attack. This is how innocent people get dragged into wars about religion.