Hi,
Some examples:
Hinduism:
The Rig Veda sets down the rules of war at 6-75:15,
Arjun
Krishna tells Arjuna that he should fight, ...
it is his duty - his dharma - to fight because he was born a warrior
he was born a member of a warrior caste and his duty to his caste and the divine structure of society are more important than his personal feelings
violence only affects the body and cannot harm the soul, so killing is not a fault and there is no reason for Arjuna not to kill people, nor should he be sorry for those he has killed.
Throughout recent history the martial function of religion was similar. A common faith served as a rallying cry in wars. This martial role of religion is very obvious for Muslims with their war cry of “Allah akbar” (God is great) and for Christians with their bellicose hymns such as “Onward Christian soldiers.” It is ironic that each of these religions bills itself as a religion of peace.
Is Religion Better at Making Peace or Making War?
Catholics and Protestants fought in Northern Ireland
To be fair religion is rarely the intrinsic cause of conflict and explicitly religious wars, such as the medieval Crusades are rare.
Nations that adhere to religions of peace may be fond of smiting their enemies but they generally do so only if there is some practical benefit to be gained.