After Bhishma withdrew himself from the position of leading the Kaurava army in the great Mahabharata war, Drona assumed his responsibilities; he was a great teacher and trainer in archery and other skills of warfare. In fact he was reverentially called Dronacharya and served as the royal Guru for both the Pandavas and Kauravas.
Before he taught anybody he used to test the student if he was worthy of receiving the warfare skills and if he belonged to the proper class and community. He used to teach only certain classes of people like the Kshatriyas who were traditionally skilled in such arts and who in those days were meant to follow such professions.
He once refused to teach a low caste student by name of Ekalavya, Ekalavya however disguised himself as a Kshatriya eligible for being taught by him. Drona was unaware of the true background of his new student and believed he was of the right caste and society, taught him all the skills successfully. After learning archery from his Guru Drona, Ekalavya passed all the examinations and proved he was excellent in the skills he had learnt.
One day when the teacher wanted to rest for a while, Ekalavya offered his thigh to be used as a pillow, but during the period of his sleep, Ekalavya was being bitten by a wasp, but he did not move lest he should disturb his master’s sleep. However when Drona woke up he saw a pool of blood under the thigh of Ekalavya and suspected that his student could not be of the caste he professed as he did not fight back the wasp which was sucking his blood. He then demanded to know the truth and Ekalavya had to confess about his caste and how he disguised himself for leaning under the great master. Drona was very angry and wanted to give him such a punishment that he would not be able to use his skills again. So, Drona demanded his normal guru Dakshina (offering) as was the practice in those days to honor the teacher with a present after successful completion of any course.
He demanded that Ekalavya give him the thumb of his right hand by which Ekalavya would become incapacitated from any further archery. Ekalavya was extremely sorry for this situation but because of his integrity he immediately cut off his thumb and presented it to his master. However Ekalavya remained a legendary figure and some communities in India, the Bhils, worship him, in some villages there are some sort of temples erected in his honour.
When Drona assumed charge of the Kaurava army, it was very difficult for his own student Pandavas to face him in war, he and his son Aswathama destroyed much of the Pandava army, like wild fire. There was panic in the Pandava’s army. Krishna, the eternal protector of the Pandavas and was fighting on the side of the Pandavas as the charioteer of Arjuna, advised the Pandavas that Drona was really invincible unless a ruse is found to trap him. Krishna explained that if he hears and believes this to be true, on any sorrowful news he would throw away his arms and cease fighting. In order that he may believe this news to be true, Krishna planned that Yudhistara, the eldest of the Pandavas should convey the message, as he was known to never utter a lie. Bhisma was then asked to kill an elephant by the name of Aswathama, the same name as Drona’s son. Immediately Yudhistara announced to Drona that Aswathama had died, whereupon upon Drona thought that it was his own son who died, so he immediately dropped his arms and retired for meditation.