Overview
Dharma-yuddha is a Sanskrit term combining dharma (धर्म), meaning righteousness, and yuddha (युद्ध), meaning warfare. In the Hindu scriptures, the term denotes a righteous or just war, one fought in adherence to a code of conduct intended to make the conflict fair and ethically grounded.
According to the tradition, a dharma-yuddha is governed by several rules of engagement. Equals are expected to fight equals: chariot warriors are not to attack cavalry or infantry, and those mounted on elephants are not to attack foot soldiers. The use of celestial weapons—divine arms bestowed by the gods—is forbidden against ordinary soldiers, being reserved for combat between warriors of noble birth. The build-up of weapons and armies is to be conducted openly, with the full knowledge of the opposing side, and no surprise attacks are permitted.
The rules also lay down how combatants are to treat non-combatants and those rendered defenceless. An enemy who has temporarily lost or dropped his weapon is not to be attacked. The lives of women, ascetics, prisoners of war, and farmers are regarded as sacred and inviolable. Pillaging of the land is likewise forbidden.
Beyond these procedural restraints, dharma-yuddha carries a moral dimension: the war must not be waged for personal gain or selfish ends. It is undertaken to uphold the principles of righteousness, and this purpose is held to distinguish it from other forms of armed conflict described in the Hindu textual tradition.
References
Adapted from the English Wikipedia article on Dharma-yuddha.