Religious fundamentalism came with monotheism. There were no holy wars before it. Sure, ancient times were cruel, wars between nations were raging, but they were motivated and justified by the economy not the ideology.
Comparing to our era, those times had an enormous religious exchange and tolerance. Different cultures explored the known world and other cultures. E.G. Egyptian pantheon was known to and influenced the Greeks. And its followers were not treated as infidels. Now, what's so different about monotheism?
First of all, monotheism is a written religion. There is always some holy book, one that explains it all. One that (they want us to believe despite the historical facts) doesn't and shouldn't change because the holy words of god are in it. Yes, that is called dogma.
photo by Esther Simpson
Against all the logic and even against common traditions of jews, christians and muslims, their dogmas differ. And they find it enough to kill each others for centuries. Sure, politics, territorial claims and plundering were, and still are, nice add-ons but the level of hatred based on the ideology alone was never seen before.
Truth to be told, polytheistic people were not all nice and pretty. The old gods had their moments as well. They started wars and fought for domination. But never the size of later monotheistic campaigns, whether they were crusades or attempts of full extermination of the other side. Romans were very passionate about wiping out christians once the movement started to spread. Though, in that case we have monotheism involved once again.
But, it's much more than just dogma, that makes monotheistic religions such troublemakers.
Unlike polytheism, where gods and goddesses are representing the forces and principles of the nature, monotheism deals with one and only god. One that is everything. God with capital G. Note the words one and everything. While the Greeks could comprehend the existence of Isis, whether just as one of the Greek goddesses with a different name or as a completely different idea, in monotheism there is no space for something other than the one and only. So every other god must be fake or an usurpator. In either case – an enemy.
Monotheistic religions are, by their nature and logic, monopolistic. They simply can't tolerate competition. If the God is everything, there's no room for anybody else.