The Americans appeared in the final year of the war. The Germans hadd been fighting since 1914, and in 1918, we war-weary, mainly young kids or battle-tired veterans, and were then suddenly faced with a fit, healthy, and well-equipped Army in the US. Plus the physical size of the US nation, and the size of the Army that suddenly arrived to support the Allies was a huge morale blow to the Germans, and shortly after the Americans entered combat, Germans started surrendering in large numbers.
When you've lived on practically starvation rations, lived in a muddy hole, probably been wounded, seen your friends killed around you, been terrified for your life nearly every day, and been eaten by lice and rats for over 3 years, a super-fit and huge army heading your way is going to make you want to give up.
