SB 1.7.19

yadāśaraṇam ātmānam
 aikṣata śrānta-vājinam
astraṁ brahma-śiro mene
 ātma-trāṇaṁ dvijātmajaḥ
Synonyms: 
yadā — when; aśaraṇam — without being alternatively protected; ātmānam — his own self; aikṣata — saw; śrānta-vājinam — the horses being tired; astram — weapon; brahma-śiraḥ — the topmost or ultimate (nuclear); mene — applied; ātma-trāṇam — just to save himself; dvija-ātma-jaḥ — the son of a brāhmaṇa.
Translation: 
When the son of the brāhmaṇa [Aśvatthāmā] saw that his horses were tired, he considered that there was no alternative for protection outside of his using the ultimate weapon, the brahmāstra [nuclear weapon].
Purport: 

In the ultimate issue only, when there is no alternative, the nuclear weapon called the brahmāstra is applied. The word dvijātmajaḥ is significant here because Aśvatthāmā, although the son of Droṇācārya, was not exactly a qualified brāhmaṇa. The most intelligent man is called a brāhmaṇa, and it is not a hereditary title. Aśvatthāmā was also formerly called the brahma-bandhu, or the friend of a brāhmaṇa. Being a friend of a brāhmaṇa does not mean that one is a brāhmaṇa by qualification. A friend or son of a brāhmaṇa, when fully qualified, can be called a brāhmaṇa and not otherwise. Since Aśvatthāmā’s decision is immature, he is purposely called herein the son of a brāhmaṇa.