Dronacharya was a childhood friend of King Dhrupad, just as Sudama was of Krishna, a poor Brahman boy and a prince befriending each other in the levelling grounds of a gurukul..the child Dhurpad promises his friend half his kingdom when he grows up.
But unlike Krishna, Dhrupad refused to recognise his childhood friend when he became king. They say he insulted him.. Dhrupad was a powerful and secure king.But maybe the paegentry of his present completely held his attention, and he was unable to accomodate that memory into it, like Dushyant, when he could remember Shakuntala, who he had loved and married in the forest, when she came to his court.
Like a child who balks when he is held to a promise he made in a moment of carelessness, Dhurpad refused.. moreover, taking advantage of his adulthood, his power, he declared that he was willing to give the Brahman alms.
Drona left the court, feeling humiliated in his learning, his poverty, his love for a childhood memory.
He was married to the sister of Kripa, the teacher of the princes of Hastinapur. Armed with the special learning of combat, given to him by Parsuram, Drona soon became the military teacher of the princes at Hastinapur.
He finally made the adult Pandvas defeat Drupad and annexed half his kingdom.
He taught the Pandavas and the Kauravas, but Arjuna was his favorite pupil. He loved his own son, Ashwathama, of course, but for his inheritance, he seemed to have chosen Arjuna over him. And over Eklavya, the young tribal boy who was the only potential competition to Arjuna, but Guru Dronacharya asked for his thumb as gurudakshina, therefore asking, in exchange for the learning he gave to his student, the possibility of his student ever being able to practise his skill.
The Mahabharata is a story that explores the spectrum of power relations that the human mind is capable of. It constructs intricate relationships in an endless structure of corridors of matches. Kinship and the fight over leadership of a kingdom, form odd patterns, old relationships have to be ruthlessly abandoned in the path of duty.
Dronacharya presided over the killing of Arjuna's beloved warrior son, Abhimanyu, even as he might have marvelled at the boy's skill at warfare so reminscent of the young boy that Dronacharya taught once. Yudhistara, who never lied, told a half lie and announced that Ashwathama was dead. He appended it with 'the elephant' but Krisha had those words drowned in the noise of battle conches.Drona paused long enough to be killed by Dristadyumna, who was born out of a sacrifical fire, invoked by Drupad when he was vanquished by the Pandavas, for the sole purpose of getting his half kingdom back from his childhood friend.
The Mahabharat happened then, in a Gurukul, in Drona and Drupad's childhood.