On US impact on WWI:
Ayers, Statistical Summary of the War with Germany, 1919
American entry into the war had two opposite effects on the French army: on one hand, it provided a great morale boost as a promise of certain victory.
On the other hand, French soldiers had bore the brunt of the war until then. The French army had been attacking all along the line from the Argonne to the sea for most of 1915, with very little actual British help. In 1916, the Somme was actually a sideshow compared to Verdun, and even then the French supplied roughly 1/4 of the troops during the initial stage. So by early 1917 and especially after the disastrous Chemin des Dames offensive, they felt that offensives now had to wait for the Americans to be present in sufficient force to take their "fare share" of the burden of attacking.
The allied plans for 1918 were to weather the expected German Spring offensive (no reason why it shouldn't be a failure after the extensive Allied experience of such outcomes) while the AEF gathered it's strength, and couterattack when the latter was ready.
The initial success of the Kaiser's battle was a great shock for the Allies, but it was defeated with little American participation. The fight of 2nd US div. at Bois Belleau was important and quite dramatic, and certainly Foch had clamoured very loudly for the div's commitment, but the first German attack was already spent.
The French recovered faster from the shock than the British, and started counterattacking on July 18 near Soisson, with IIRC roughly 10% of the troops supplied by the AEF. This battle (2nd Marne) lasted until August 6th and succeeded in recapturing all the ground lost to the German offensive in the French sector. It wasn't until 2 days later that the British started counterattacking in their sector at Amiens with IIRC 2 US divs in support.
The next significant US battle was St-Mihiel. Now this wasn't exactly a brilliant victory for the AEF, as the Germans had scheduled a withdrawal from this salient starting the day after the AEF offensive started, and so just conducted an almost pre-planned fighting retreat. To say they were defeated is a bit far-fetched.
THE really significant involvement in the ground war was the Meuse-Argonne part of the general Allied offensive, at the end of which the AEF supplied just under 1/3rd of the attacking force.
Their capacity for attacking was actually less than that, for the AEF was quite deficient in artillery compared to the British and more especially the French.
By November, 1918, the AEF had an artillery park of slightly more than 3,000 guns, mostly field and medium artillery. The French had a bit more than 10,000, half of which were heavy artillery. And of the 3,000 AEF guns, 95% had been supplied by the French.
The figures for Aircraft were roughly the same.
Even the munitions fired by US guns were mostly supplied by French production (@ 60% IIRC).
Now most allied shells were filled with US-produced HE, and by Autumn the US were supplying a significant proportion of the chemical weapons used to fill British and French gas shells - although again, I don't think a single US-made gas shell was ever fired during WWI, owing to difficulties with starting production).
The reason for this was mostly the almost total lack of preparation for war in the US. The gear-up went incredibly fast, and had the war lasted into 1919, the US would probably have started supplying the British and French with guns, tanks, shells, etc, but the war ended to early for US industrial mobilization to play any significant role and for the US-built high-tech stuff to reach the front.
The oil question was raised in an earlier post. I don't remember about figures, but the US supplied most of the oil used by the French during the war.
So, had the US never entered the war, what would have happened?
There's no reason the US would have stopped supplying the allies with oil and powder.
I can't see how the outcome of the German offensive would have been significantly different.
The Allies would have counterattacked, although with less troops at their disposal, progress would have been much slower and the German morale collapse less dramatic without US units in the line.
So in all probability the war would have lasted into 1919, by which time social and economic pressures in Germany would have brought about a revolution, or at least enough unrest to bring Germany to ask for terms.
will naturally try to boost his already large ego by telling everyone how great it is, while the other animals try and downplay this to assuage their bruised egos. 
