Given the amount of casualties I can't believe that this was an easy task? Were they just the final push that was needed but couldn't be provided by French and British forces? Did they introduce new tactics that helped?
The key contribution of American forces was their potential IMO. American forces became increasingly involved with the Hundred Days Offensive, however the British and French forces were still the major contributors to this even after being stretched in facing down the Spring Offensives.
I mean potential in the sense that with the failure of the Spring Offensives, the Germans were now faced with the reality of facing growing and fresh (if inexperienced) American forces backed by an expanding war economy. Had the war continued in 1919, US armies would most likely have played an increasingly important role given their increasing manpower and relative exhaustion of the British and French manpower reserves.