Of an agreement with Stalin before Hitler could. I’m not sold that would have been productive, as Stalin might have been able to play GB and Germany against each other. And given the choice between a deal with GB protecting Poland’s sovereignty, or one with Hitler giving Russia a piece of the conquest of Poland, who really thinks Stalin would have sided with GB?

Bottom line, the Brits and French made all sorts of mistakes in the run up to WW2 - that’s why they were so unprepared in 1938. It wasn’t just Chamberlain’s trust in Hitler after Czechoslovakia. Just that he should not have assumed that would be the end of Hitler’s aggression. He all but engraved an invitation to Hitler to invade Poland, who had good reason to expect France and England to continue to do no more than complain.