Karna was a son of a charioteer by his foster father and if I am not wrong he didn't fight with the previous king of Anga.
When the Pandavas and Kauravas returned to Hastinapur after completing their education, they were showing their skills infront of the whole kingdom. When Arjun had almost defeated Duryodhan, Karna fired a bow from the crowd and saved Duryodhan and challenged Arjun. When he told his identity to everyone, he was refused to compete with Arjun as he was from a lower caste and son of a charioteer. Seeing this Duryodhan gifted him Anga Pradesh (so that he becomes a king and thus eligible to fight with Arjun). Source: This is the conversation between Arjun and Karna can be found in the Sambhav Parva
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