Manu himself, the son of the Self-create (Brahman), has said that battles should be fought fairly. The righteous should always treat the righteous fairly. They should follow righteousness without destroying it.
तस्माद् धर्मेण योद्धव्यमिति स्वायम्भुवोऽब्रवीत्॥ सत्सु नित्यः सतां धर्मस्तमास्थाय न नाशयेत्।