Bhagavad Gita Verse 2.33

किन्तु यदि तू इस धर्मयुक्त युद्धको नहीं करेगा तू तो स्वधर्म और कीर्तिको खोकर पापको प्राप्त होगा|

By not fighting this war, O ARJUNA, you are committing a sin because you fail to perform your duty as a warrior and you also will lose your honour.