Śrīmad-Bhāgavatam · Text 1.7.19
Who is speaking and story frame
यदाशरणमात्मानमैक्षत श्रान्तवाजिनम् ।
अस्त्रं ब्रह्मशिरो मेने आत्मत्राणं द्विजात्मज: ॥ १९ ॥
yadāśaraṇam ātmānam
aikṣata śrānta-vājinam
astraṁ brahma-śiro mene
ātma-trāṇaṁ dvijātmajaḥ
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].
Read Śrīla Prabhupāda's purport on Vedabase.io