For years I've been defining a religious war as a battle to the death, among thousands or more people, over whose imaginary invisible friend is more powerful.
I'm not sure it's as clear-cut as that (perhaps for example is labeled as religious, but the real reasons for engaging in it are not all as frivolous as arguing over nonsense.... and perhaps it is wrong to make light over people motivated by their understanding of moral goodness who are willing to fight for what they think is right even if their related mythologies and metaphysics have nothing to do with reality) but still it's perhaps a useful pithy rejoinder to anyone seeking to go in the direction of using lethal violence just to impose and validate one's views.
One time in particular I revisited this concept was after 9-11 when there was so much passion for the US and UK taking the fight to the enemy, not just where the enemy was, but drumming up support for fighting in other countries in the Middle East that didn't have anything directly to do with the 9-11 attack.
My thought was that we've been fighting the Crusades for 1000 years, and they were (from what little I know) a terrible idea then, and they're certainly a terrible idea now (to the extent that any modern day efforts in the Middle East may in some quarters be highly motivated by religion). Here again it's arguably bad to be sloppy and over-simplify, so I'll admit there may be good reasons in some cases to go and fight, but to the extent that they're based on arguing over nonsense, they're terrible reasons to shed blood.