By all estimates, Kunti discovers that Karna was the son she abandoned when the latter arrives in open court on the day that all the Kuru princes were to display the skill they’d learnt during their time in training under Drona.
RELATED READ: The little known story of Karna’s generosity
Karna shows up demanding that he be given an opportunity too. However, since the display required the warrior to be of a royal lineage, Karna gets shot down. On seeing this Duryodhana immediately announces that he would grant Karna a region called Anga and make him its king. In doing so Duryodhana secures Karna’s lifelong loyalty and Karna finds in Duryodhana a lifelong friend.
Kunti watches this entire episode with a sense of foreboding, knowing that at some point in the distant future, her firstborn would most likely come face-to-face with her other sons. Even so, Kunti chooses to keep the secret to herself.
Throughout the years, Kunti watches Karna’s reputation grow even as her own sons continue to shower him with disdain. Never once does she come forth to reveal the true identity of Karna.
Ultimately, Karna learns of the truth from three different people.
When all talks to prevent the war fail, Krishna, who’s negotiating on the behalf of the Pandavas takes Karna to the side and tells him that only he had the power to stop the war. He explains to Karna that he wasn’t an ordinary charioteer’s son but rather the son of a god and a princess. And that the princess who is now a queen had five sons just like him. Karna realises that it was Kunti that was his birth mother and is shattered. Even so, he refuses to stop the war.
RELATED READ: Was Karna a better archer than Arjuna?
The day before the war commences, Kunti approaches Karna and tells him the truth behind his birth. She begs him to reconsider and says that all his brothers would follow his lead since he was the oldest. Karna refuses yet again, pointing out that not only had she abandoned him at birth, she also didn’t stand up for him over the years when he was ridiculed and not given his due simply because the world believed he was born to a charioteer.
Karna proceeds to point out that his real mother was the one who raised him and not one who abandoned him. At which point, Kunti asks that he spare the lives of her sons. Karna agrees but says that the courtesy wouldn’t extend to Arjuna who had been his nemesis. Kunti also manages to get him to promise that he would only use the divine weapons once, thus reducing his arsenal size considerably.
Karna also learns of his birth through Bhishma but smack in the middle of the battle. He goes to Bhishma to seek his blessings as he rests on the bed of arrows made by Arjuna when the grandsire reveals Karna’s true origins. By all estimates, Bhishma had known about this for a long time and most likely through Ganga, his mother. For it is in the Ganges that Kunti abandons the infant Karna. In the weeks preceding the war, Bhishma humiliates Karna in open court which angers the young king who refuses to participate in the war till Bhishma falls. In another version of the story, Bhishma says he won’t fight alongside Karna since he wasn’t a royal by birth forcing Karna to stand down even though he wanted to fight. It can be said that Bhishma, knowing of Karna’s origins, did all this to prevent him from facing off against his brothers.
Of course, all these attempts to make Karna stand down or switch sides fail as he feels obligated to Duryodhana more than to the biological family that abandoned and ridiculed him his entire life.