Complex sentence: On the 13 September, Pont-à-Mousson and Lunéville were recaptured by the French unopposed and the French forces advanced until close to the Seille river, where the front stabilized until 1918.
Split into simpler sentences: On the 13th, Pont-à-Mousson and Lunéville were recaptured by the French without combat. The French forces continued to advance until close to the Seille river, where the front stabilized until 1918.