OK let's look at pre20th Century
The wars of the ancient world were rarely, if ever, based on religion. These wars were for territorial conquest, to control borders, secure trade routes, or respond to an internal challenge to political authority. In fact, the ancient conquerors, whether Egyptian, Babylonian, Persian, Greek, or Roman, openly welcomed the religious beliefs of those they conquered, and often added the new gods to their own pantheon.
Medieval and Renaissance wars were also typically about control and wealth as city-states vied for power, often with the support, but rarely instigation, of the Church.
And the Mongol Asian rampage, which is thought to have killed nearly 30 million people, had no religious component whatsoever.
Going back to the modern wars ;
The American Revolution, World War I, World War II, Vietnam, Korea; none of these wars was fought for religious reasons.
In fact, the bloodiest and most deadly wars of recent history were demonstrably motivated by something other than religion:skewed nationalism (WWI), antireligious fascism (WWII), and atheistic Communism (Korea, Vietnam, the atrocities of Stalin and Mao).
Ultimately, religions don’t go to war. Governments do, and they usually must convince an ambivalent populace of their decision to do so. In this, they may use religion as a motivating factor (whether or not the religion of the enemies is different), but that doesn’t make religion the cause of war.
Often wars are fought when there is no difference in religion. In the bloodiest war in U.S. history, the Civil War, the North and the South had the same religion.