What caused France and great Britain to end their appeasement and declare war on Germany?

France and Great Britain ended their policy of appeasement and declared war on Germany following its invasion of Poland in September 1939. The violation of Polish sovereignty, coupled with Germany's aggressive expansionism under Adolf Hitler, prompted both nations to realize that further concessions would only embolden Nazi aggression. The failure of appeasement to prevent conflict and the need to uphold their commitment to Poland ultimately led to their declaration of war on September 3, 1939.