Bhagavad-Gita is one of the holy books of Hindus and worlds largest reading book. On it, Lord Krishna makes a speech to Arjuna where Arjuna is in a religious war and fought against the iniquity of Kauravs who was their cousin-brothers. And when Arjun felt guilty to fight against their relatives.
Then Lord Krishna convinced Arjuna to fight for the truth. And give him(the human beings) advice.
Then Lord Krishna replied
if you like to read more about mythology then here some for you,
Lord krishna on Arjuna at the war field
11 BRIEF LESSONS FROM THE BHAGAVAD GITA FOR BRINGING PEACE INTO OUR LIVES