Just because religious people fight doesn’t make their wars necessarily religious. For example: let’s say a Catholic and a Baptist live next to each other, and one day they get in a fight. Is it a religiously motivated conflict?
Well, it all depends. For example, they may be fighting over the fact that the Baptist put up a fence and it infringed on the property the Catholic believed belonged to him. Rather than ask for the survey of the property they decide to duke it out. That’s not religious.
But if they come to blows because the Baptist said disparaging things about Catholicism… that would be a religious conflict.
It has occurred to me that the first conflict recorded in Scripture was a conflict over power and control – not religion (the 5 kinds attack on the cities of Sodom and Gomorrah). In fact, many of the conflicts recorded in Scripture had as their focus power and control. The first major “religious conflict” was between Egypt and Israel (where Israel actually did no fighting). But 40 years later Israel invaded the Promised Land at God’s behest… that was a religious conflict. And so were the constant conflicts with Israel and the Philistines.
But, strictly speaking, the conflicts of Assyria, Babylon and Egypt (while they may have religious overtones, were focused again on power and control. As were the wars in Persia, Greece, Carthage and Rome.
The same could be said for most of the conflicts in the Far East.
The next serious religious conflict was between the followers of Islam and their neighbors. Islam went to war as a religious objective, and while power and control were part of the mix, religion took a higher place on the agenda.
Many people look at the Crusades as principally religious in nature, but even a couple of those were not always about religion… but about control and power (such as the conflict between the Crusaders and defenders of the Byzantine Empire).
In Europe, there was a mixed bag of religion and power. The Thirty Years’ War (for example) was a series of wars in Central Europe between 1618 and 1648. It was one of the longest, most destructive conflicts in European history. Initially a war between Protestant and Catholic states in the fragmenting Holy Roman Empire, it gradually developed into a more general conflict involving most of the great powers of Europe, becoming less about religion and more a continuation of the France–Habsburg rivalry for European political pre-eminence.