Reply #3: Crimey - where do you want to start?

[enlightenment]

As mentioned, the Crusades.

The French Wars of Religion - those were really fun. Not.

The Thirty Years War.

The Reconquista.

The Troubles (N.Ireland).

umm - smaller scale - Henry VIII's purges, followed by daughter Mary's purges, followed by sister Liz's purges.

The English Civil War was essentially a religious war, dressed up in some fancy secular clothes.
Monmouth's Rebellion was purely an attempt to remove a Catholic king from the throne.

Religion of whatever stripe is the best excuse ever to kill other people and get away with it!