Complete financial collapse in Britain and France before the end of the year 1917, inability to keep industrial war machine going, severe undersupply of Anglo-French military forces, and food shortages in both countries, especially Britain. Presuming that the Russians ceased to be a serious military threat as well - not an unreasonable assumption - the Germans would have met negligible resistance on the Western Front and probably could've secured a peace treaty including Ludendorff's
sine qua non, Belgium.
I've made multiple posts on this, and also incorporated the idea into the Eurasian War alternate history I wrote once upon a time.
This post discusses it somewhat, and led into a broader discussion that included
this one. More facts there.