Complex sentence: Germany could have won the Battle of the Marne, they think, if only Bülow had matched the courageous initiatives of Kluck's Army, although this doesn't explain the near encirclement of his army.
Split into simpler sentences: Germany could have won the Battle of the Marne, they think, if only Bülow had matched the courageous initiatives of Kluck's Army. Although this doesn't explain the near encirclement of his army.