Religion and war, are they always related with each other?

I can't really say it is religion that causes war. Usually it is man's flawed perception of his religion and his blinding pride and arrogance that often fuels tensions. Sometimes, in the name of religion, a group would believe another group to be evil because they don't believe exactly as they do and they often seek to exterminate the other. Many religions in their purest form call for people to live at peace, love, and humility with each other.

No there has been a lot of conflict in this world that's had nothing to do with religion. Not all religious conflicts end with war either. As long as man is greedy in nature there will always be wars.

Not all religious conflicts cause wars, and not all causes of war are religious. Wars are waged for economic and idealogical reasons too. Capitalism vs Communism wasn't a religious war, and many other wars are waged for money and resources. The problem with saying "religion causes wars" is, you're also ignoring the good it has done. Anything that causes differences can cause conflicts leading to violence, and religion is no exception. But, well, religion also inspires people to do humanitarian work. Religion is also the basis of our moral code. Look at the now-extinct cannibal cultures: Without a religion that says "Killing and eating people is wrong", that sort of thing happens because it's not considered inappropriate. Religion brings both good AND conflict. People have to learn to accept the differences to prevent conflict from leading to violence. If they can do that, the good will be all that's left.