Historians have given widely different interpretations of Chamberlain’s offer of a guarantee to Poland and of its significance. The evidence for Chamberlain believing it was no longer possible to deal with Hitler on the same basis as at Munich is examined, as is the evidence that the prime minister was increasingly constrained in foreign policy by public opinion, which had come round to Churchill’s position on appeasement. Churchill urged a reluctant Chamberlain to back the guarantee with an Anglo-Franco-Soviet Alliance, and the prime minister has been much criticised for failing to do so. The failure is explained with reference to intelligence assessments of Soviet capability and reliability and also to Soviet demands in the negotiations and to Stalin’s decision to opt for a pact with Hitler. The significance of Chamberlain’s secret contacts with Germans and Swedes in the summer of 1939 is explored. Chamberlain’s conduct in the final crisis in August and September is considered in the light of claims made then and also by some historians that he was forced by the House of Commons into declaring war.