If Poland is on the verge of being conquered by the Soviets, then I have no doubt that German forces would occupy the corridor, Posen and Upper Silesia. If not by the Reichswehr itself, then certainly by Freikorps, with silent acquiescence of the German government. They would probably claim that it’s necessary to protect their eastern flank from the Bolsheviks (and it would be true in some ways), which France or Britain would find difficult to argue against. I mean, what would be the alternative? Whether the Germans occupy their pre 1914 territories in the east or not, the young, independent nation of Poland would cease to exist anyway. Unless France and Britain allow Germany to rearm to liberate Poland (which I very much doubt), the facts on the ground in Poland aren’t going to change anytime soon. If anything, many of the Poles living in formerly German territories would probably prefer to be under German jurisdiction again, then to be occupied by the Bolsheviks.
The question is, what happens afterward? With Poland conquered, where do the Soviets go next? Unless they decide to actually invade Germany, their most likely (and easiest) targets would probably be the Baltic states, which would be kind of a thorn in their side now that they’ve taken Poland. But would the western powers allow that? Or would they try to intervene?
