What was the immediate event that prompted Britain and France to declare war on Germany?

The invasion of Poland was the immediate event that prompted Britain and France to declare war on Germany in September 1939. This act marked a significant escalation in Germany's aggressive expansionist policies under Adolf Hitler, as it represented a direct violation of the Treaty of Versailles and the principle of collective security that had been established after World War I.

Prior to this, Germany had already annexed Austria and the Sudetenland and occupied Czechoslovakia, actions that had gone largely unchallenged by major powers, partially due to policies of appeasement. However, when Germany invaded Poland, it crossed a critical line. Britain and France had both guaranteed Poland's borders and sovereignty, and thus felt compelled to take military action. The invasion on September 1, 1939, was swift and brutal, leading to the fall of Poland in a matter of weeks.

The other events listed, such as the annexation of Austria, the occupation of Czechoslovakia, and the signing of the Munich Agreement, were contributing factors to the tensions leading up to the war but did not directly trigger the declarations of war by Britain and France. Instead, those events exemplified the growing threat posed by Hitler and the failures of appeasement, ultimately