The brahmastras are finer than the nuclear weapons. Asvatthama discharged the brahmastra simply to kill the Pandavas, namely the five brothers headed by Maharaja Yudhisthira and their only grandson, who was lying within the womb of Uttara. Therefore the brahmastra, more effective and finer than the atomic weapons, was not as blind as the atomic bombs. When the atomic bombs are discharged they do not discriminate between the target and others. Mainly the atomic bombs do harm to the innocent because there is no control. The brahmastra is not like that. It marks out the target and proceeds accordingly without harming the innocent.