If the question is about Hitler turning up the aggression in Poland
instead of the annexation of Czechia in or around March '39 it might not involve the Allies; after all they only stopped with the appeasement once he invaded lands that didn't have lots of Germans in them.
My predictions: Allies do nothing initially and it comes to a shooting war between Poland (like hell are the Poles ceding territory willingly at this point
) and Germany.
With the Wehrmacht less well prepared, no or later Soviet intervention in the East (probably an after-the-fact treaty to appease Stalin with historical or near-historical territorial division) and maybe a more complete mobilization in Poland, the war lasts longer, say 3-4 months, until the Poles are defeated, and the Wehrmacht's casualties are probably over 200k all told with >60k dead (OTL, Poland was the Nazis' firs campaign and they actually did horribly for casualties given the vast superiority they had - in excess of 50k casualties out of which almost a third died outright).
Hitler being Hitler occupies it all, not just the border claims (and gives Stalin the east), and the Allies are furious at him for it. If they don't declare at once to take advantage of the Wehrmacht being (bloodied) in Poland, they will the next time Hitler puts a half a toe out of line.