The US military intervention came at a pretty critical time. The German Spring Offensive in 1918 was cutting a pretty large swath into allied territory. Their new stormtrooper tactics allowed them to make some relatively substantial gains at first. Were they going to be capable of fully breaking through and winning the war with this offensive? Most likely not. Germany was running critically low on manpower and their population was suffering as a result of the blockades they had been under for years. The offensive was their last ditch effort to win the war.

That being said, the first substantial contingents of US troops started arriving on the line at just the moment when the German offensive looked most threatening. The US troops were green and lacked the same level of artillery support/coordination of the other powers, but an infusion of fresh, eager soldiers into the fight proved to be a huge morale boost to the British and even moreso to the beleaguered French. At that time, there weren't enough US troops in France to form their own separate army group, so US units fought attached to those of the British and French. They fought hard and the other allies admired their tenacity, but they were very inexperienced and suffered high casualties. What they learned fighting in the spring they put to excellent use during the Hundred Days Offensive.

Where the US really made their mark was during the aforementioned Hundred Days Offensive that followed the failure of the German's Spring Offensive. Germany did not have the capability to replace the losses of men and material that they suffered during the spring of 1918. They almost certainly would have collapsed regardless of whether the US intervened or not. The British and French at this point were much better off than the Germans, and though they took many casualties, they pretty much rolled over the German lines. The US just sped up the process.

As mentioned before the spring offensive of 1918 was a point where Germany at least seemed to get close to a final victory in the war. The only other real point where the Germans were close to this victory was during it's first offensives in Belgium and France. German troops bashed trough the French and Belgian defences and came so close to Paris that it's citizens could hear the German guns roar. Meanwhile the British did not yet have a large enough army on the continent to make a real difference and the Belgian army was almost driven from it's own country. This situation was saved by a number of factors; the Germans lost connections with their troops and commanders (commanders didn't have a clue where their troops where), their logistics were seriously overstretched and maybe most importantly, the French, just like the allies in 1918 did not retreat in chaos. Instead Joffre and his subordinates maintained discipline and with that the French army itself. Because of this the French didn't lose many guns and at the most critical point, when the Germans were most overstretched, were able to strike back and in coordination with their allies decisively beat the Germans at the battle of the Marne.