Final answer:
Britain and France promised assistance to Poland when Germany invaded in 1939, failing to protect Poland and later grasp the severity of the war threat, resulting in the Phony War period.
Step-by-step explanation:
Britain and France promised to help Germany if it was attacked during World War II in the framework of their mutual defense agreements with Poland. When Germany invaded Poland on September 1, 1939, Britain and France saw this as a violation of Poland's sovereignty and a direct threat to European stability. Despite the declaration of war on Germany, Britain and France could not provide immediate effective military support to Poland. Underestimating the potential duration and severity of German aggression, they had hoped the Polish resistance, coupled with their own slow mobilization, would suffice.
Later, however, the Phony War showed the limitations of French and British military strategy. They were unable to prevent the rapid conquest of Poland by the combined forces of Germany from the west and the USSR from the east. The subsequent fall of Norway and Denmark and the crushing defeat of France within weeks exposed the folly of the Allied pre-war tactics and strategizing. Their reticence to engage and earlier failures to enforce the Treaty of Versailles emboldened Hitler's expansionist ambitions, ultimately plunging Europe into a devastating war.