Prior to Christianity, and the unrelenting rejection of all other faiths which defines the Judaism from which it arose, religious war was, in my understanding, unheard of. The Romsns let everyone do what they wanted, and for those offering incense and sacrifices they felt kinship, regardless of the name of the God or Goddess on the other side of the phone line. This held even for the Jews, who until the destruction of their Temple were still slitting animals throats and casting them into fires.
This is what was so confusing about Christisnity–no sacrifices, and absolute intolerance. That spirit of course infused Islam, where the most dogmatic aspects of Christian intolerance were elevated to positive commandments.
But we need to remember that “religion”, per se, was not a reason for warfare for much of human history.