In chapters 7-12 of The Once and Future King, conflict arises between Arthur and Lancelot: Arthur is married to and deeply loves Guenevere, but she loves Lancelot, who is deeply in love with her as well. She is found guilty of adultery and is sentenced to be burned at the stake for treason. Lancelot rescues her, and there is a fight; but a truce is declared and Guenevere returns to Arthur. Gawain still wants revenge because Lancelot killed his brothers. So Arthur and his knights follow him to France and lay siege to his castle.
With the king out of England, and out of his way, Mordred decides it is his time to strike. He tells Guenevere that he plans to announce to the people that Arthur has been killed in France and that Mordred is now the king. Guenevere manages to get a message about this to Arthur, who decides to return to England immediately to reclaim his throne. He figures that Mordred is more of an enemy to him than Lancelot is.