Wikipedia
Dharma-yuddha is a Sanskrit word made up of two roots: dharma meaning righteousness, and yuddha meaning warfare. In the ancient Indian texts, dharma-yuddha refers to a war that is fought while following several rules that make the war fair.
For instance, in a righteous war, equals fight equals. Chariot warriors are not supposed to attack cavalry and infantry, those on elephants are not supposed to attack infantry, and so on. The rules also forbid the usage of celestial weapons (divine weapons bestowed by the gods) on ordinary soldiers (as opposed to soldiers of noble birth). The build-up of weapons and armies is done with the full knowledge of the opposing side and no surprise attacks are made.
The rules of engagement also set out how warriors were to deal with noncombatants. No one should attack an enemy who has temporarily lost or dropped their weapon. The lives of women, prisoners of war, and farmers were also sacred. Pillaging the land was forbidden.
Dharma-yuddha also signifies that the war is not fought for gain or selfish reasons. A dharma-yuddha is waged to uphold the principles of righteousness.