SB 1.7 - Arjuna counteracts the brahmāstra
Arjuna asked wherefrom the dangerous effulgence, which was spreading all around, was coming from. Anything that is presented before the Lord should be done so after due presentation of prayers, like Arjuna did. Kṛṣṇa replied that the son of Droṇa had released the brahmāstra but did not know how to retract it. The subtle science of releasing weapons, done by chanting mantras, is still material. A chanter of hymns knew how to apply the weapon as well as how to retract it. Since Aśvatthāmā did not know how to retract the weapon, what he did was irreligious. Kṛṣṇa said that only another brahmāstra could counteract the weapon, and asked Arjuna to subdue it with the power of Arjuna’s own weapon.
Hearing this from Kṛṣṇa, Arjuna touched water for purification, and after circumambulating the Lord, cast his brahmāstra to counteract the other one. When the rays of the two brahmāstras combined, a great circle of fire covered all outer space and the whole firmament of planets. It was like the combination of the sun and the fire emanating from Saṅkarṣaṇa’s mouth at the time of devastation. The radiation of atomic energy is very insignificant compared to the heat produced by a brahmāstra. The former can utmost blow up one globe, but the latter can destroy the whole cosmic situation. The heat generated by brahmāstra resembles the fire exhibited by the sun at the time of cosmic annihilation. Although the brahmāstra was released on this earth, the heat produced by the combination of both weapons covered the three worlds.
Seeing the disturbance of the general populace and the imminent destruction of the planets, Arjuna at once retracted both the weapons, as Lord Kṛṣṇa desired. Without the Lord’s sanction the world cannot be destroyed, for the natural laws work under His direction. By the Lord’s will only, the many powerful agents of the Lord can execute His desires.