Final answer:
Britain and France entered World War II following Germany's invasion of Poland in 1939, which broke the hope that appeasement policies would prevent further German expansion.
Step-by-step explanation:
Britain and France were drawn into war with Germany due to a series of aggressive actions taken by Adolf Hitler and the German army. Notably, the invasion of Poland by Germany on September 1, 1939, served as the immediate catalyst for Britain and France to declare war on Germany, marking the beginning of World War II. Prior to this, both countries had followed a policy of appeasement, hoping to avoid another large-scale war by conceding to some of Germany's demands, such as the annexation of parts of Czechoslovakia after the Munich Pact. Despite efforts to contain Hitler's expansionist agenda, it became clear that appeasement would not stop further German aggression, which became evident with the subsequent occupation of the rest of Czechoslovakia.