Before people start fighting me over the subject, I first want to say that actually I am a Christian and I go to church every Sunday and participate in many of the church's activities. However, I am really convinced that if we go back the moment humans were created on this earth, and take away every religion, there will be world peace... I don't know about you but I would really love to know your opinions.
This is complete and utter guff. No one has ever gone to war over religion. People always go to war over resources. Any war you care to mention, resources.
- Ancient Greeks would dress up wars in Honour. Rescuing an abducted King's wife or avenging a slaughter. But bashing Troy from example is a good way to take out your largest trading rival.
- Crusaders would fight for Christianity but they'd be looting the riches of the middle East. This is evidenced by the Pope ordering the 4th crusade to attack the Christian city of Zara.
- Even the second world war was about resources. Churchill said long before Hilter invaded Poland "Germany shall have a war whether she wants one or not", and of course Hitler famously did not want a war with Britain. But it was dressed up as liberating Poland. Germany attacked Poland etc because it needed resources (food production to fuel its industrial empire) and Britain attacked Germany because German industry was dominating European trade taking their market share. Neither side gave a damn about the Jews.
The people who fight in wars are rarely the ones who profit from it, and so they must be told something that will inspire them to fight.
What is religion for? Well religion is one of the 3 pillars of civilisation. No civilisation has ever amounted to anything without a religion. The 3 pillars are ...
- Sound money
- Rule of law
- Religion
You can't build a civilisation without all 3 and if you remove any of the three, you get the end of empire.
I'll focus on religion for this answer though. Religion is good for people. You either have a big religion or a big government. You can't have neither. Trey Gowdy does an excellent speech illustrating the correlation.
Goto 6 minutes and 17 seconds.
If you have a big government, you have few freedoms. Government will seek to maintain order by being pervasive and infiltrating every aspect of your life. And this will cost you a fortune in taxes as a big government has huge costs. But a religion creates order without government. Good religions all have an aspect of law in them. Thou shalt not kill, thou shalt not steal etc and they also all share an omnipotent God and an after life. Both are important. When no one else is watching you, God is. And if you aren't caught committing crime by a man, there will be divine retribution when you die. And this creates order. A set of rules that everyone can live by that don't need massive state intervention. Big religion, small government.
Now the argument many atheists have (and they are wrong), is that they have their own morality and everyone else should too. The problem with this is that people define their own rules and some people are dumb. There is an excellent video where an atheist woman is challenged that animal lives are worth the same as human lives. Sorry but I can't find it.
And she argued that she had the moral high ground and animals were every bit as important. And the rebuttal was that the bible said human life was sacrosanct. Not animal life. And she scoffed and trashed the mans beliefs. So he asked her what her morality was built on if not religion. And she replied love and compassion.
So he asked her "If you were walking your dog and both your dog and a stranger were stood on a frozen lake, and the ice cracked giving you time to save only one of them, which one would you save? The dog you love, or the stranger you know nothing about?" And she fucking paused. She thought about it for about 20-30 seconds by which time the guy asking said you've taken so long they are both dead.
So she says "my dog, I'd save my dog".
And that is the reason we have religions. It takes the guess work out of making important decisions of morality. She would save a wretched dog to give it 5 more years of life because she would act selfishly and she prizes love and animal life above that of men. Rather than suffer a little personal sense of loss, she would let a parent lose their son, a man lose his brother, a woman lose her husband and a child lose its father. Some people are just fucking dumb, and religion is good for them. If she thought for one second her own mortal soul would spend an eternity burning in hell if she didn't help the man, she'd have got him out pronto, because you can bet your last dime she values HER life more than that of her dog's.
So a set of moral rules everyone shares is important to building a successful civilisation. you cannot argue against that. There have been no successful civilisations that were atheist. None.
Now atheists like to criticise a religion by semantically picking holes in a story or part of it. Adam and Eve is a favourite refuge of the atheist. How can the first man and woman have 3 sons and mankind continue? Anthropology is not what the story is about. It is about resisting temptation. And religious people haven't just been stupid throughout the ages. There is this misconception that people were stupid before 1900 and that they are much smarter now and don't need religion. Ask one of those people to design a Cathedral however and they might appreciate that our forefathers weren't that stupid. Religious people ignore the flaws in a religion. It is called blind faith. You are blind to the flaws because you know that if you and everyone else buy into it, your lives will be immeasurably better. Would you rather know the truth and live in misery or believe in the fantasy if it means your life is better. Terry Pratchet understands this concept and Death explains it in the Hogfather.
Goto 1 minute and 51 seconds.
As Death says, you need to believe in things that aren't true to be human. To be civilised. It is no coincidence that when Europeans tried to civilise Africans and South American natives, they sent missionaries.
So having made my case that religion serves a purpose and that without it we would not be able to be civilised let me explain why religion is used to justify war.
Because it is so powerful. It is one of the 3 pillars. If you break any of the pillars you destroy a civilisation ... (yes the west is in decline because it removed usury laws forbidden by religion). It is now drowning in debt. Religion is there to hold the law to account. So when unscrupulous lawmakers are lobbied and changed the law against religion the people should cry heresy! Even the suggestion should be blasphemy. But by the time usury laws were revoked you had "educated" men leading Deist movements and claiming God was only for the stupid. Religion could no longer be the defense.
If you can grab any of the three pillars, you can rule a nation. Kings tend to grab hold of law. And steer a nation with its laws. Nathan Rothschild grabbed the money supply and boasted "I care not who makes the laws in a nation, he who controls the money supply controls the British Empire and I control the money supply". If we'd had an absolute King of course that one person could have just outlawed that, but Britain had a parliament and parliaments are full of corruption. The Vatican derived its power and wealth by grabbing hold of religion.
Anyone who abuses one of the three pillars is someone we define as a tyrant. Henry VIII is the only tyrant I can think of that abused all three. A super tyrant. He split religion and changed it so he could get a divorce to ensure his own linage. (fortunately only a small change which damaged the religion but didn't destroy it, most was left intact). He counterfeited the nation's money reducing the silver pound sterling from 92.5% silver down to 42.5%. Fortunately his daughter Elizabeth I put it right back so Britain didn't die in the same way the roman Empire did by counterfeiting the Denarius until it had 1/500th the silver it purported to have. And he changed all kinds of laws taking land from people and destroying private property rights. A grade A villain.
And so it is not the religion that is the problem. It is the men who take a religion and command its people to do awful things in the name of God. But without the religion there could be no civilisation. And if men could not control a religion, they would just seek to control the money or laws of a nation instead. It doesn't make any difference. They will cause wars whenever they need resources and blame whatever they can get away with. Unfortunately religion is a common goto. But the other two can do the same. Rothchild's mother said on her death bed, "if my sons did not want war, there would be none".
So the tl;dr answer to the OP is, people never go to war for religion. They go to war for resources, and religion is a handy excuse, but a different excuse would be conjured if religion didn't exist.