Three young German POWs after the Battle of the Marne, 1918.
This here exemplifies Germany’s defeat in the First World War; man power shortages. It may come as a surprise to some that in early 1918 the Germans held a ~300,000 man advantage on the Western Front, before the Americans could arrive in force. This is due to 1) The transfer of 50 divisions to France from the Eastern Front after Russia left the war, 2) Germany having a considerably larger man power base than France or Britain, but not both combined, and 3) attrition working in Germany’s favor consistently since 1914. But with the U.S entry into the war in 1917, Germany would soon find herself overwhelmed by the allies if she did not score a quick victory within a year or so. That puts us right in April 1918, Operations Michel and Georgette were stunning tactical victories, but strategic failures, and the elite sturmtruppen fell in droves as a result. In total, tens of thousands of sturmtruppen, ceased to be by July 1918, along with hundreds of thousands of regulars, with nothing to show for it. This is where the Americans come in, and the man power advantage swings back towards the allies as Germany’s situation only gets worse, to the point where young boys and old men see action where they most certainly shouldn’t, Germany was essentially bled white. This is where the main U.S contribution to the war comes in. It was little about what the doughboys and Marines actually did in the trenches (although it was notable in and of itself) but the strategic and operational constraints levied on the German Army from April 1917 onwards. Had the U.S stayed neutral, Germany would not have been forced to show its hand like it did in March 1918, and with the manpower advantage they enjoyed, along with their stout defenses along the Hindenburg Line, a negotiated peace would take place between Germany, and the allies, likely favoring Germany. Keep in mind the situation in France and Britain wasn’t so peachy either, and if Germany could keep up their positive rates of attrition, I don’t see how Germany could have lost as bad as they did with American involvement. Of course defeat is still possible, just less likely.