By 1917 both Britain and France were being 'tapped out' for manpower. The BIG enlistments from the initial rush to join up had been decimated in the battles of 1916 at Verdun and the Somme. New replacements were only managing to fill gaps, as opposed to providing fresh rroops in large numbers or offensive use. The last French offensive in 1917 ended in a mutiny that jeopardised future plans to attack the Germans.
When the Russian Empire collapsed in 1917, the Germans freed-up about 1.5 million soldiers for service on the western front. They were used in the spring offensives of 1918 which occurred at about the same time the American army was taking the field. At the Marne, in 1918, it was a Franco-American force that stopped the last German drive on Paris.
New American forces took the place of some French and British, allowing a respite for training and reorganization that made the last great offensive, in late summer and fall, 1918 effective.