People tend to massively overestimate the contribution of religion to human conflict throughout history due to popular narratives and availability bias.
It's been estimated that religion has been the primary motivation in somewhere between 6 and 9% of wars (and this includes the Arab conquests which, imo, were more imperial than religious). Religious divisions consistently map to other ethno-cultural-linguistic divisions too, which makes identifying 'religious' conflicts even more difficult.
With 91-94% of human conflict being non-religious, it's clearly not one of the top reasons why conflict occurs though.
In addition, religion unites as well as divides. Many people tend to assume absent religion there would be more unity, but this is certainly not self-evident as humans are naturally divided as a species and religion has been one of the most unifying forces in human history. Logically, this unifying force must have prevented an unknown number of conflicts also.
Another issue is that people tend to compare religious conflict against a baseline of zero. So if, hypothetically, there were 1000 religious wars, they assume that absent religion there would have been 0 wars between these states. The problem is if you remove religion you create a vacuum that must be filled by a different guiding ideology/belief system. We have no way of running an alternative history with dozens of new ideologies to see how many wars these caused. If we look at all kinds of societies across human history though, there is no reason to believe they would have been predominately peaceful.
Ultimately, we have no way of knowing religion's net contribution to violence in terms of violence fomented and violence prevented. At worst it has caused 9% of wars (and a smaller percentage of deaths), but, in theory, could have actually been responsible for a net decrease in violence although we can never know this one way or the other.
Many people (not you, I'm generalising about 'New Atheist' types) want to blame religion as they see violence as some kind of error or distortion of human nature (inhuman animals!) and thus need to identify the culprit. Many Humanists (as belies their Christian heritage) see religion as filling the role of the devil, with Reason (i.e Divine Providence) set to deliver us from evil in their secular salvation narrative. This is why their view of religion in history is completely out of step with the views of historians and secular academics and is more a secular version of evangelical Protestant preaching than it is the impartial and reasoned analysis they purportedly favour.