Bhagavad Gītā · Chapter 1 · Arjuna Viṣāda Yoga
कुलक्षये प्रणश्यन्ति कुलधर्माः सनातनाः
धर्मे नष्टे कुलं कृत्स्नमधर्मोऽभिभवत्युत
॥ १.४० ॥
kulakṣaye praṇaśyanti kuladharmāḥ sanātanāḥ|
dharme naṣṭe kulaṃ kṛtsnamadharmo'bhibhavatyuta||
Word meanings
कुलक्षये = when the family is destroyed
सनातनाः = ancient
कुलधर्माः = family traditions
प्रणश्यन्ति = perish
उत = and
धर्म = dharma
नष्टे = when destroyed
कृत्स्नम् = entire
कुलम् = the family
अधर्मः = unrighteousness
अभिभवति = overwhelms
Translation
Age-long family traditions disappear with the destruction of a family; and virtue having been lost, vice takes hold of the entire race.
Commentary
The commentators seated around this verse

Rāmānuja · Viśiṣṭādvaita
EnglishOn verses 1.26–47

Jñāneśvar · Vārkarī
English40 “On the extinction of the family there perish the family’s immemorial rites-and-customs (Dharma); and when the rites-and-customs perish, the entire family is overcome by impiety. (243)
Fire is produced by rubbing together pieces of wood and when it gets ablaze the entire wood is gutted: in the same way, when there arises jealousy, resulting in the mutual slaughter of family members (गोत्री) great sin is created and the whole family gets burned. Where such sins, leading to family destruction, take place, the family’s traditional religion disappears and there grow up in the family anti-religious tendencies.
Swami Ramsukhdas · Modern
English'Kulakṣaye praṇaśyanti kuladharmāḥ sanātanāḥ'—In a war, there is destruction of family. The family has its age-long traditions and time-honoured usage. But when a family is destroyed, its pious traditions and usage, which lead the living and dead members of the family to benediction, also perish, because no one remains, to maintain these.
'Dharme naṣṭe kulam kṛtsnamadharmo'bhibhavatyuta'—When pious traditions and pious conduct, perish, people lose their virtues and righteousness, and impiety takes hold of an entire family.
Now the doubt arises, how impiety takes hold of the family when it is destroyed. The answer is, that when people who wage war are killed, impiety takes hold of the children and women, who do not take part in battle, but are left behind. The reason is that when fit and experienced persons die in battle, no one is left behind to control the children and women, and to teach them good conduct and virtuous behaviour. So they do not behave decently that is supposed to mark, righteous persons. So impiety takes hold of them.

Jñāneśvar · Vārkarī
Marathiजैसें काष्ठें काष्ठ मथिजे । येथ वन्हि एक उपजे । तेणें काष्ठजात जाळिजे । प्रज्वळलेनि ॥ २४३ ॥
तैसा गोत्रींची परस्परें । जरी वधु घडे मत्सरें । तरी तेणें महादोषें घोरें । कुळचि नाशे ॥ २४४॥
म्हणवूनि येणें पापें । वंशजधर्मु लोपे । मग अधर्मुचि आरोपे । कुळामाजि ॥ २४५ ॥

Jñāneśvar · Vārkarī
Hindi(1.243) जैसे लकड़ी पर लकड़ी रगड़ने से ऐसी अग्नि निकलती है कि जो प्रदीप होते ही सब लकड़ी को जला डालती है,
(1.244) वैसे ही गोत्रजों में यदि परस्पर मत्सर के कारण वध हो तो जो घोर पाप होता है उससे कुल का ही नाश हो जाता है।
(1.245) इस पाप से कुल का कर्म लुप्त हो जाता है और कुल में अधर्म का संचार हो रहता है।

Sri Udiya Baba · Modern
Hindiकुल का नाश होने पर कुल के सनातन (परम्परागत) धर्म नष्ट हो जाते हैं और धर्म का नाश होने पर सम्पूर्ण कुल को अधर्म दबा लेता है।
Swami Ramsukhdas · Modern
Hindi‘कुलक्षये प्रणश्यन्ति कुलधर्माः सनातनाः’—जब युद्ध होता है, तब उसमें कुल-(वंश-) का क्षय (ह्रास) होता है। जबसे कुल आरम्भ हुआ है, तभीसे कुलके धर्म अर्थात् कुलकी पवित्र परम्पराएँ, पवित्र रीतियाँ, मर्यादाएँ भी परम्परासे चलती आयी हैं। परन्तु जब कुलका क्षय हो जाता है, तब सदासे कुलके साथ रहनेवाले धर्म भी नष्ट हो जाते हैं अर्थात् जन्मके समय, द्विजाति-संस्कारके समय, विवाहके समय, मृत्युके समय और मृत्युके बाद किये जानेवाले जो-जो शास्त्रीय पवित्र रीति-रिवाज हैं, जो कि जीवित और मृतात्मा मनुष्योंके लिये इस लोकमें और परलोकमें कल्याण करनेवाले हैं, वे नष्ट हो जाते हैं। कारण कि जब कुलका ही नाश हो जाता है, तब कुलके आश्रित रहनेवाले धर्म किसके आश्रित रहेंगे? ‘धर्म नष्टे कुलं कृत्स्नमधर्मोऽभिभवत्युत’—जब कुलकी पवित्र मर्यादाएँ, पवित्र आचरण नष्ट हो जाते हैं, तब धर्मका पालन न करना और धर्मसे विपरीत काम
करना अर्थात् करनेलायक कामको न करना और न करनेलायक कामको करना रूप अधर्म सम्पूर्ण कुलको दबा लेता है अर्थात् सम्पूर्ण कुलमें अधर्म छा जाता है। अब यहाँ यह शंका होती है कि जब कुल नष्ट हो जायगा, कुल रहेगा ही नहीं, तब अधर्म किसको दबायेगा? इसका उत्तर यह है कि जो लड़ाईके योग्य पुरुष हैं, वे तो युद्धमें मारे जाते हैं; किन्तु जो लड़ाईके योग्य नहीं हैं, ऐसे जो बालक और स्त्रियाँ पीछे बच जाती हैं, उनको अधर्म दबा लेता है। कारण कि जब युद्धमें शस्त्र, शास्त्र, व्यवहार आदिके जानकार और अनुभवी पुरुष मर जाते हैं, तब पीछे बचे लोगोंको अच्छी शिक्षा देनेवाले, उनपर शासन करनेवाले नहीं रहते। इससे मर्यादाका, व्यवहारका ज्ञान न होनेसे वे मनमाना आचरण करने लग जाते हैं अर्थात् वे करनेलायक कामको तो करते नहीं और न करनेलायक कामको करने लग जाते हैं। इसलिये उनमें अधर्म फैल जाता है।
Sanjaya frankly speaks to Dhritarāṣṭra at this time saying that Arjuna, who is the most noble minded, the most compassionate, the eternal well-wisher and the most virtuous;
although frequently fraught with difficulties along with his four brothers, instigated by thee and thy son, including horrid acts of treachery against them with intent to kill,
such as attempting to burn them all alive in a specially made house of combustible lac, administering poison and other unworthy deeds as well.
It is surely known to you that by Arjuna the destruction of all your clan is certain due to the fact that he has the support of the Parama Puruṣa, Krishna Himself, by his side.
He felt weak, overcome by his love and extreme compassion for his relatives. He was also filled with fear, not knowing what was Dharma(righteous) and what was Adharma (unrighteous). His mind was distracted by grief at the thought of losing his relatives.
So he cast aside his bow and arrow and sat down on the floor of the chariot as if to fast to the death.