Concerning World War 1, the arrival of US troops did not make an enormous difference. They were certainly a fresh and motivated force that more than likely helped shorten the war by being able to keep up a level of zeal not seen since 1914; however, the greater allied force did have a Spring Offensive planned for 1919, which is certainly evidence that they planned for the US to not be a dealbreaker.

As for World War 2, it’s more difficult to say concerning North Africa and Europe. I won’t touch the Pacific since it’s rather self-explanatory, and Europe is the point of contention. Some could argue that the US was an aid in North Africa and shut out Rommel’s last hopes, or that the simple mass of men the US could throw into the theater in France was what won the war, but the subject is still debated to this day.