Final answer:
Great Britain and France declared war on Germany in 1939 in response to Germany's invasion of Poland, which was the breaking point after failed appeasement efforts and a mutual defense agreement with Poland.
Step-by-step explanation:
The direct cause of Great Britain and France declaring war on Germany in 1939 was the invasion of Poland by German forces. On September 1, 1939, Germany invaded Poland, falsely claiming that Poles had been mistreating ethnic Germans. The invasion was the final straw for Britain and France, underscoring the failure of appeasement and the realization that Hitler could not be trusted. Two days after the invasion, Britain and France honored their commitment to Poland and declared war on Germany, marking the beginning of World War II in Europe. However, despite their declarations, the Allies did little to aid Poland directly, which swiftly fell to the combined forces of Germany from the west and the Soviet Union from the east.
Previously, Britain and France had pursued an appeasement policy, hoping to avoid another catastrophic war by making concessions to Germany. They had also signed a mutual defense agreement with Poland, seeing the threat of German aggression. The nonaggression pact between the Soviet Union and Germany also played a role by allowing a coordinated attack on Poland without fear of Soviet intervention against Germany. Sadly, this resulted in the crushing of Poland and ignited the fires of World War II in Europe.