The brahmastra cannot be counteracted by any weapon except another brahmastra. Arjuna had counteracted one brahmastra with one of his previously. Now Asvatthama had released five separate brahmastras for each of the Pandavas. This was difficult to counteract because it would take time to release five brahmastras. Considering (viksya) the unavoidable danger, though the Lord had renounced all weapons, he protected his devotees, even if he had to break his own promise, in order to preserve his unique quality of having affection for his devotees.