Complex sentence: Next day the Fifth Army recrossed the Marne and the German 1st and 2nd armies began to retire as the French Ninth, Fourth and Third armies fought defensive battles against the 3rd Army which was forced to retreat with the 1st and 2nd armies on 9 September.
Split into simpler sentences: Next day the Fifth Army recrossed the Marne, and the German 1st and 2nd armies began to retire. The French Ninth, Fourth, and Third armies fought defensive battles against the 3rd Army, which was forced to retreat with the 1st and 2nd armies on 9 September.