Complex sentence: Wartensleben, in the center, was held up by French riflemen until his third (reserve) detachment arrived to outflank them; the French retreated across the rivers, destroying all the bridges.
Split into simpler sentences: Wartensleben, in the center, was held up by French riflemen until his third (reserve) detachment arrived to outflank them. The French retreated across the rivers, destroying all the bridges.