Steffen what you say is tru of the land battle....
but don't forget about the war at sea, where the US contribution probably began by the summer of 1917.
No doubt about it Britain was vastly navally superior to Germany and
capable of blockading it. However, the effects, and even the
effectiveness of the blockade itself grew over time.
The blockade still had leaks in early 1917. The entry of the United
States, and its Navy however, almost instantly contributed to
air-tightening the blockade. Blockade patrols were reinforced, and
more importantly, Britain and the USA totally clamped down on the
transit of goods to Germany through the Nordic states and Netherlands.
This had already been greatly restricted, but until the spring of
1917, some low level of commerce had been permitted to avoid
irritating the most important neutral, the US. In other words, the Allies became more hard-ass towards Denmark at this point.
Also, interned German and Austrian shipping in the US, which had been
sitting idle in American ports, became available for Allied use in the
spring of 1917.
Finally the American Fleet became ready by the summer of 1917 to
suggest convoys, an idea the British Admiralty had been resisting
until then (in WWII by contrast, the US was the dullard on convoys and
the British had the right idea first), and once decided, was able to
help convoys, reducing the U-Boat threat and helping get supplies
across.
So, although it took over a year for the US to have even a
quantitative impact on the ground war, it much more quickly made a
difference in the war at sea.
So, if you remove the US from the equation, who have the possibility
of the Germans importing some goods from overseas during 1917 and
1918, you don't have free interned shipping, and convoys may not start
as early or be as effective, leading to higher losses from U-Boats.
[Assuming of course, the PoD is not a German decision to abstain from
U-Boat warfare, but instead the passage of the act keeping US
passengers off belligerent ships. If the PoD involves Germany simply
abstaining from U-Boats and Zimmerman, the looser blockade still
benefits them and the Allies still don't get interned shipping, but
the Allies are better off than in the first option].
So, what will be the cumulative effects of a somewhat worse Allied
supply situation [Yes all sophisticated militarty hardware came from
France and Germany, but bulk rations and so on came from overseas]and
a somewhat better German supply situation by spring 1918? Especially
if Mike Stone is correct that in OTL the winter of 1917-1918 was
actually not as bad as that of 1916-1917 for Germany?