This story is about how Lord Krishna saved the earth from imminent destruction. This was due to a weapon, Brahmashira, which was far more potent that Brahmastra.
Brahmashira – the weapon more potent than Brahmastra
All of us hear about the supreme weapon Brahmastra. This is known to be the mother of all weapons and has been talked about in quite a few instances. Such as Lord Rama, intending to dry the ocean, Meghnad using this on Hanumana in Lanka just before the Lanka Dahan.
But few of know that Brahmashira was even more potent than Brahmastra. But, every person, who possessed the knowledge of Brahmashira , was strictly forbidden by his teacher on grounds of humanity. This weapon has power the to possibly wreak more havoc than simply annihilating an entire army.
So we do not hear the use of this weapon in mythological stories despite a number of persons knowing this, including Drona, Bhishma, Arjuna, Karna and Ashwathama (son of Drona).
Karna’ greatness is again underlined here. He, for the sake of humanity, did not used this against Arjuna, despite knowing this very well. But Ashwathama did not hesitate to use this.
Ashwaththama does the unpardonable act
So Ashwathama, at once stage of the war, while facing Arjun, had picked up a grass straw and uttered the key phrase of invoking this mighty astra and had flung it towards Arjun.
Arjun, on realising the imminent danger threw at Ashwathama an arrow invoking his own Brahmashira so as to neutralize it; but Lord krishna knew that a Brahmashira cannot be neutralized, unless revoke by the one who has used it, and that collision of two Brahmashira would result in an earthly-collapse. So, he asked the duo to revoke their respective astras.
Arjuna could revoke the weapon but Ashwathama could not, so Lord Krishna had to intervene and save the earth from an imminent danger.