Why were the French and British troops relieved when the Americans arrived in the western front?

The question “Why were the French and British troops relieved when the Americans arrived on the Western front?” is assumed to relate to the First World War.
 
The generic and obvious answer is that one would expect any fighting force to feel at least a sense of hope and enthusiasm at the arrival of substantial help. Relief, on the other hand, presumes that the French and British were anticipating a negative outcome which the arrival of the Americans was likely to change. In broad terms, the arrival of American troops in the Western Front during World War I signaled a material change in the balance of forces allied against the German led Central Powers, ultimately ensuring that they would be defeated.
 
More specifically, the arrival of American troops on French soil in 1917 promised that desperately needed manpower would soon be on the way to replace the horrible casualties suffered by the French and British during the two primary battles of 1916, namely the Somme and Verdun. Additionally, in 1917 Russia plunged into civil war as a result of the Communist revolution there, an event which ultimately led to the withdrawal of Russia from combat with the Germans. This meant that Germany could redeploy substantial numbers of seasoned troops to the Western Front for a final push to defeat France and Britain there. Campaigning in 1917 was inconclusive, and by the early 1918 only 85,000 US troops had arrived, but the resources of the US were massive and needed only time to be brought to bear. By the time of the final battles and the end of the war in November, 1918, over 1.2 million US troops had arrived to participate.