Lesson No 9
Exploit your opponents'weakness and take calculated risks.
Krishna was very smart. He sent Yudhishthira just before the battle to the Kaurava's camp to take blessings from his elders. He won them over with this move and they all gave him the secrets of Kauravas'army. He also played the trump card by inviting anyone who wanted to change sides and Yuyutsu, Dhritrashtra's son came to his side exposing the non-cohesiveness of Kauravas.
Kauravas didn't know their opponents'weaknesses hence couldn't do much about it. Duryodhana was so proud that he cared less about his army.