Everybody realised that Hitler's next target was Poland, so Chamberlain promised to defend Poland. But how? Britain was too far away to help and only had a small army.
In April 1939, Stalin suggested an alliance of Russia, France and Britain against Germany. Hitler would not have been able to invade Poland if taking that action would have meant war with Russia.
However, negotiations dragged on into August because:
Out of the blue, on 23 August 1939, Hitler made the Nazi-Soviet Pact with Stalin - a promise not to go to war with each other and (secretly) a promise to invade Poland and split it between them.