TRANSLATION

When the son of the brahmana [Asvatthama] saw that his horses were tired, he considered that there was no alternative for protection outside his using the ultimate weapon, the brahmastra [nuclear weapon].

In the ultimate issue only, when there is no other alternative, the nuclear weapon called the brahmastra is applied. The word dvijatma-jah 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 an 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 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.