Complex sentence: Reaching the Aisne in under six hours, the Germans smashed through eight Allied divisions on a line between Reims and Soissons, pushing the Allies back to the river Vesle and gaining an extra 15 km of territory by nightfall.
Split into simpler sentences: Reaching the Aisne in under 6 hours, the Germans had smashed through 8 Allied divisions on a line between Reims and Soissons. They had pushed the Allies back to the river Vesle and had gained an extra 15 km of territory by nightfall.