Reading Notes: Mahabharata Part D

 The Pandavas had a plan created I which Drona would no longer be able to fight thus they would be able to kill him. The plan was for Bhima to notify Drona that his son passes away. However, before they even told Drona that his son passed away, they named an elephant Ashwatthama which was named after Drona's son and they murdered it. This basically made what they were telling Drona somewhat true. Drona didn't know this was a set up therefore hearing of his sons death distracted him in which the Pandavas were able to kill him. 

Drona: Wikimedia

Source: Story 154: Drona Hears Word of Ashwatthama's Death

Bibliography: Laura Gibbs, Tiny Tales from the Mahabharata