Dronacharya left Drupada’s kingdom and vowed to take revenge against him. Kripacharya, the brother of Kripi and the guru of Dhritarashtra, suggested to Dronacharya that he should seek employment in Hastinapur. Dronacharya agreed, and, along with his family and Kripacharya, left for Hastinapur.
After reaching Hastinapur, Drona encountered the Kuru princes and helped them retrieve their ball from a well by tying together stalks of grass. When Bhishma found out about this, he was impressed and appointed Dronacharya as the guru of the Kuru princes (the Kauravas and the Pandavas).
Dronacharya agreed to train the Kuru princes on the condition that, after their training, they would defeat the King of Panchala, Drupada, as Guru Dakshina (a custom where the student offers something to the teacher in exchange for knowledge).
The Kuru princes, along with Ashwatthama, were trained in Dronacharya’s ashram in Hastinapur. All the princes excelled in the use of various weapons of warfare.