Śrīmad-Bhāgavatam 1.8.13

The brahmāstra, the supreme weapon released by Aśvatthāmā, was something similar to the nuclear weapon but with more radiation and heat. This brahmāstra is the product of a more subtle science, being the product of a finer sound, a mantra recorded in the Vedas. Another advantage of this weapon is that it is not blind like the nuclear weapon because it can be directed only to the target and nothing else. Aśvatthāmā released the weapon just to finish all the male members of Pāṇḍu's family; therefore in one sense it was more dangerous than the atomic bombs because it could penetrate even the most protected place and would never miss the target. Knowing all this, Lord Śrī Kṛṣṇa at once took up His personal weapon to protect His devotees, who did not know anyone other than Kṛṣṇa. In the Bhagavad-gītā the Lord has clearly promised that His devotees are never to be vanquished. And He behaves according to the quality or degree of the devotional service rendered by the devotees. Here the word ananya-viṣayātmanām is significant. The Pāṇḍavas were cent percent dependent on the protection of the Lord, although they were all great warriors themselves. But the Lord neglects even the greatest warriors and also vanquishes them in no time. When the Lord saw that there was no time for the Pāṇḍavas to counteract the brahmāstra of Aśvatthāmā, He took up His weapon even at the risk of breaking His own vow. Although the Battle of Kurukṣetra was almost finished, still, according to His vow, He should not have taken up His own weapon. But the emergency was more important than the vow. He is better known as the bhakta-vatsala, or the lover of His devotee, and thus He preferred to continue as bhakta-vatsala than to be a worldly moralist who never breaks his solemn vow.