The entering of the US into World War I was a psychological boost to the allied forces because they know had more soldiers/resources to help them win. By this time, many of the European countries had been at war with the Axis powers for three years. By the time the US enters in 1917, many European countries have lost hundreds of thousands of soldiers. Along with this, their current troops are mentally and physically exhausted by the war.
Now that the US has joined, they are able to rely on them for soldiers who are fresh and ready to go. This changes the outcome of World War I significantly.
