Otto II convened the Imperial Diet in mid-July at Dortmund. There, Otto II declared war against France and prepared his army to march west. In September 978, Otto II retaliated against Lothair by invading France with the aid of Charles. He met with little resistance on French territory, devastating the land around Rheims, Soissons, and Laon. Otto II then had Charles crowned as King of the Franks by Theodoric I, Bishop of Metz. Lothair then fled to the French capital of Paris and was there besieged by Otto II and Charles. Sickness among his troops brought on by winter and a French relief army under Hugh Capet forced Otto II and Charles to lift the siege on November 30, and to return to Germany. On the journey back to Germany, Otto's rearguard was attacked and destroyed by French forces, with their supplies being captured. Despite neither side obtaining a clear victory, Otto II felt his honor was sufficiently restored and opened peace negotiations with the French King. Peace was finally concluded between Otto II and Lothair in 980: in return for renouncing his claims on Lorraine, Otto II would recognize Lothair's son Louis V as the rightful heir to the French throne.