One verse

Srimad Bhagavatam 1.7.19

1.7.19

यदाशरणमात्मानमैक्षत श्रान्तवाजिनम् । अस्त्रं ब्रह्मशिरो मेने आत्मत्राणं द्विजात्मज: ॥ १९ ॥

yadasaranam atmanam aiksata sranta-vajinam astram brahma-siro mene atma-tranam dvijatmajah

yada
when
asaranam
without being alternatively protected
atmanam
his own self
aiksata
saw
sranta-vajinam
the horses being tired
astram
weapon
brahma-sirah
the topmost or ultimate (nuclear)
mene
applied
atma-tranam
just to save himself
dvija-atma-jah
the son of a brahmana.

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).