Cross-posted from my answer to the question: Why is Lord Krishna called the master of finesse in the Mahabharat? on Quora
I will attempt to answer this, but my answer is going to be controversial. Please keep in mind that it is not my intention to offend anyone.
Alright, let’s start with some ground rules. For the length of this answer, consider that Krishna is not Lord Krishna, the god and that the Mahabharat is not a religious text revered by millions. Instead, let us analyze the Mahabharat as if it is a (ridiculously good) fantasy story, and Krishna is a central character in this story.
Introduction and back-story:
Krishna is introduced to us early on, as a cousin of the Pandavas. He is accomplished, wise, and powerful in his own right (unlike Pandavas or Kauravas at this point, who are squabbling children). This is because he has overthrown Kamsa, the King of Surasena kingdom, native land of the Yadavas. However, he has multiple problems of his own. Firstly, he is a cowherd, not royalty. The Yadavas rule by council, not by king. Thus, even though Krishna is the leader of the Yadavas, he is not equal to royalty. Further, Kamsa was Jarasandh’s Senapati. Jarasandh is the emperor of India. Every other dynasty, including the Kurus, pays obeisance to him. This doesn’t leave Krishna with very many allies. That Krishna is capable and a political genius (at such a young age) is evident because Jarasandh has been unable to crush him outright. But Krishna has now fought 18 wars with Jarasandh, and has slowly lost territory to him. He has been forced to leave his land and move to the island of Dwarka. He needs help.
Guess who else need help? The Pandavas. They are staking a claim to the throne of Hastinapur on extremely untenable grounds: It is an open secret that Pandu was impotent. Who then, are these children, arrived from nowhere? Here is Krishna’s opportunity. The Pandavas will oppose Duryodhana, who is Dhritarashtra’s son. Dhritarashtra is politically aligned with Jarasandh. Thus, Pandavas are going against Jarasandh himself. Krishna aligns himself with the Pandavas.