Bhagavad Gita

ದೋಷೈರೇತೈಃ ಕುಲಘ್ನಾನಾಂ ವರ್ಣಸಂಕರಕಾರಕೈಃ ।
ಉತ್ಸಾದ್ಯಂತೇ ಜಾತಿಧರ್ಮಾಃ ಕುಲಧರ್ಮಾಶ್ಚ ಶಾಶ್ವತಾಃ ॥೪೩॥
doṣairetaiḥ kulaghnānāṃ varṇasaṃkarakārakaiḥ |
utsādyaṃte jātidharmāḥ kuladharmāśca śāśvatāḥ ||43||

Sloka 1:43
Gist of the sloka:
Those who have no responsibility of their actions destroy the society, clans, its proper dharma and destroys its very existence.
War destroys youth, leaders, creates imbalance between men-women. To set right the status it might take unknown number of years. By proceeding to the war, we might be the cause of such actions. By destroying the balance in the society, we might disturb the society, its dharma along with the natural growth and evolution of one’s character [swabhava]. Sanathana dharma itself might be hurt. This is the purport of Arjuna to Lord Krishna.
Mahabharatha war caused over 50 lakh people to lose their lives. This represented almost 10% of the population of the country at that time.