What led France and great Britain to declare war against Germany to begin world war 2?

France and Great Britain declared war against Germany after Adolf Hitler invaded Poland on September 1, 1939. Both countries had previously pledged to protect Poland's sovereignty through mutual defense agreements. Their declarations of war on September 3, 1939, were driven by a desire to stop German aggression and uphold their commitments to Poland, marking the official start of World War II. This action reflected the failure of appeasement strategies that had allowed Nazi expansion to go unchecked.