What effect did american soldiers have on the war?

In the short term, America's entry very nearly caused a German victory. The Germans knew that once American troops arrived in force, they would overwhelm the Germans. Therefore they decided to launch one last massive offensive to gain victory before America could transport enough troops to Europe. In Spring 1918, this oofensive was launched in the Kaisershlecht (Kaiser's Battle). This massive offensive was ver succesful, brought the Germans almost within Gunning range of Paris, and very nearly knocked Britain out of the war in mainland Europe. fortunately for the allies it was halted and turned back just outside paris. 

In the Long term, America's entry had the opposite effect. Both sides were exhausted and the arrival of fresh, optimistic American troops gave Britain and France the extra incentive needed to gain an allied Victory. However, America did not bring about an allied victory. By the time America entered, germany was suffering under the British Naval Blockade, which was depriving them of adequate food and military equipment. Had America not joined the Allies almost certaintly would have won anyway. In reality, America's entry probably shortened the war by a few months, maybe a year.

hope i helped