In WW1, there have been many attempts at breakthroughs in order to evade the trench system. The battle of Somme was one of the less successful attempts at a German breakthrough. Yet, before USA managed to get to the scene of the war Germany also made a breakthrough, yet there not able to optimize this breakthrough. When the triple entente and the USA finally broke through, they managed to optimize it very well and countered Germany right back to Mons. I was wondering why was the triple entente able to take back so much more land than the Germans once they counter-attacked?
The tide turned pyschologoically at the (Second) Battle of the Marne, where the Germans reached their "high water mark" before being turned back by soldiers of the U.S. Third Division (a later "version" of this same division distinguished itself in the Iraq War). The psychological impact was comparable to the arrival of the Prussians at Waterloo. Basically, the Allied troops knew that they were "backstopped" and could fight with confidence, and the Germans knew that they were "alone."
The newly-arrived American troops represented less than a 20% addition in manpower to the Allies, but a far greater addition in overall Allied fighting strength because they were "fresh." This more than made up for their "inexperience" in trench warfare. Besides, Pershing didn't want his troops fighting in the trenches. It was the Germans who had invented ways to "cirncumvent" the trenches (e.g. with storm groups), but the German-American Pershing took that to a new level by having Americans fight "wilderness" style, on their hands and knees.
In this case "Breakthrough" is less accurate than "Breakdown". Even before the Americans arrived the Germans had far fewer men covering each mile of front than the French + British. In fact, at that point in the war the Germans only wanted a victory that would push the war into peace talks and give them a strong bargaining position. With the failure of the German's "Last gasp" offensive that hope disappeared.
The reason the Allies were able to effect such a large breakthrough is manifold, but the main reasons are lack of men and lack of food on the part of the Germans. Even before the war Germany wasn't able to produce enough food to feed all its people. When the war started and the British blockade began hunger and starvation became commonplace in Germany as well as the army. An estimated 700,000+ German civilians died as a result of the blockade. In this sense the German's final offensive actually worked against them. When the Germans began overrunning French and British trenches they found copious amounts of food and supplies, exactly the opposite of what German propaganda had been telling them. For the first time in years it became obvious to the average soldier that Germany was losing the war. Thus, when the Americans allowed the Entente to continue their offensives, German resistance crumbled.
It's also worth nothing that the Germans themselves largely blame the tank. One German officer wrote that his men, whose training and sense of duty was enough to keep them fighting against infantry, had no problems surrendering to tanks. While this is true, the larger reality of it was that Germany was simply spent at that point. They didn't have the men, they didn't have the money, and they didn't have the food to continue the war any longer. So rather than a "breakthrough" what you are seeing is the final breakdown of the German military machine across all fronts.