In 16th Century France there was a succession of wars between Roman Catholics and Protestants (Hugenots primarily). These series of wars were known as the Wars of Religion.
In the first half of the 17th century, the German states, Scandinavia (Sweden, primarily) and Poland were beset by religious warfare. Roman Catholicism and Protestantism figured in the opposing sides of this conflict.
Not unlike the Hatfields and McCoys where fighting and death occurred. But not declared war within (civil) or between countries. I think we're confusing violent religious intolerance with warring nations.
Sorry, I don't agree with your opinion that religion is the crux for histories bloodiest wars.I could list dozens more, continuing right up to the present day, all over the world......
Somebody already mentioned that religion is "sold" as pretext for war. It plays on our logic and reasoning abilities when we're led to believe that religion is the basis for conflict.
Religion was a way of getting man to follow (other men.) Whether it be salvation or pay day, many confuse the sale as legit.