astram brahma-siro mene
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
When the son of the brahmana [Asvatthama] saw that his horses were tired, he considered that there was no alternative for protection outside of his using the ultimate weapon, the brahmastra [nuclear weapon].
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
In the ultimate issue only, when there is no alternative, the nuclear weapon called the brahmastra is applied. The word dvijatmajah is significant here because Asvatthama, although the son of Dronacarya, was not exactly a qualified brahmana. The most intelligent man is called a brahmana, and it is not a hereditary title. Asvatthama was also formerly called the brahma-bandhu, or the friend of a brahmana. Being a friend of a brahmana does not mean that one is a brahmana by qualification. A friend or son of a brahmana, when fully qualified, can be called a brahmana and not otherwise. Since Asvatthama’s decision is immature, he is purposely called herein the son of a brahmana.