Guru Drona was an exemplary judge of character and capabilities. He shrewdly judged all his students and taught them in the way they needed to become the best version of themselves. He did not give the all- powerful Brahmashira to Dharmaraja or Bhimasena despite Yudisthira's sense of justice or Bhima's archery skills. Nor to Dushasana or Vikarna though they were skilled archers too. He tested the students, AGAIN, before he gave the Brahmashira to Arjuna finding that he had the control to avoid using the Brahmashira frivolously.
BUT, the same Drona failed when he gave it to Ashwattama on his insistence. In Sauptika Parva, when Ashwattama has killed the Up-pandavas and fled away, Bhima follows in a blind rage. Krishna immediately urges Yudishtira to follow because he has already relazied what Ashwattama will do when cornered. He will use the Brahmashira and that would cause terrible destruction.
It is at this point that Krishna reveals how Ashwattama had approached him during the Pandava exile adn wished to exchange his Brahmashira for the Sudarshana chakra. Krishna offered not only Sudarshana but his gada, dhanush etc etc too. But Ashwattama could not even budge the chakra an inch from its place.
(Krishna offers the same Sudarshana to Arjuna during Kurukshetra, meaning he can actually wield it but still it never occurred to him to ask Krishna for it.)
Krishna, being who he is, does not miss the opportunity to wax eloquent about his soul brother here:) :)
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