Both Astras and Shastras were earned after intense tapasya and their usage came along with strict discipline and control over the senses with one being allowed to use them only in the most unavoidable circumstances given the fact that the most powerful ones could be used just once. Each had a predefined purpose and a counter Astra too, except the rare undefeatable ones like Brahmastra. In fact, the great war of Mahabharata, it is believed was to return the divine Astras and Shastras to their rightful places since kaliyug was approaching and men with their inflated egos would have spelled doom for the Creation by deploying these Astras.

Lord Krishna devised various ways to prevent the warriors from firing the Brahmastra, which would have brought the creation to an end. For instance, Karna was cursed that when the time comes he would forget all that he had learnt. Thus he could not use Brahmastra against Arjuna. Similarly, when Dronacharya was about to deploy it against Arjun, various Gods and Goddesses came down to request him not to. Likewise when Ashwathama and Arjun did use it against each other, both were asked to take it back. Arjun took it back but Ashwathama did not know how to, so his Brahmastra was redirected by Lord Krishna to Parikshit who was in the womb of Uttara, wife of Abhimanyu. (Later Parikshit was brought back to life by Lord Krishna). In this way all the Brahmastras were returned back.