My initial thought was:
What about a russiawank in WWI, but that was asking to much.
So I went for no German victory at Tannenburg, and what could happen afterwards.
Imagine GErmany pulling out of East Prussia, because the GErman high command knows it connat be hold.
They move back to the Weichsel, the Russian Army followes.
Once the Weichsel is the frontline, it is hard to force a crossing for either side.
This could go on for years...
Any thoughts?