Eklavya's father was commander in Magadh's army, Jarasandh was ruler of Magadh, He was enemy of Mathura and Hastinapur because Kansa was his son in Law, one of the many reason why Dronacharaya cut his thumb..

Eklavya was short tempered, he shot a dog just because he barked at him. Archers were supposed to be calm & patient. Eklavya used to shoot using his Thumb that was good for hunting but not for war, there is a theory that Drona cut his thumb so he can learn to shoot with fingers

And he did learn to shoot with finger grip and later got a post in Jarasandh's army and got slained by Shri Krishna in war..