Complex sentence: The Germans came to their aid by forming up the 9th Army and attacking during the Battle of the Vistula River, and although it was initially successful, the attack eventually petered out and the Germans returned to their starting points.
Split into simpler sentences: The Germans came to their aid by forming up the 9th Army and attacking during the Battle of the Vistula River. Although it was initially successful, the attack eventually petered out and the Germans returned to their starting points.