In the Udyoga Parva of the Mahabharata, before the war, Krishna offers Karna a place on the side of the Pandavas. Karna refuses. “I cannot venture, O Janardana, to behave falsely towards the intelligent son of Dhritarashtra,” he says in the Ganguli translation. “I have, O Krishna, enjoyed sovereignty for thirteen years, without a thorn on my side.”
Later his mother, Kunti, makes the same request. Karna answers her: “How can I render that friendship of Dhritarashtra’s sons utterly futile?”