Complex sentence: Germany had freed an extra 1.7m men for military service, and by the summer of 1917 the German Army would be an extra 1.25m men stronger, an extra 89 divisions (albeit reduced from 12 infantry battalions to 9).
Split into simpler sentences: Germany had freed an extra 1.7m men for military service. By the summer of 1917 the German Army would be an extra 1.25m men stronger, an extra 89 divisions (albeit reduced from 12 infantry battalions to 9).