What caused France and great Britain TO END THEIR POLICYof eppeacement and declare war on Germany?

France and Great Britain ended their policy of appeasement and declared war on Germany primarily due to Germany's invasion of Poland on September 1, 1939. This aggressive act violated previous agreements and demonstrated that Adolf Hitler's expansionist ambitions could not be contained through diplomatic concessions. Both nations recognized that failing to act would embolden Germany further, leading to greater threats to European stability and security. Consequently, on September 3, 1939, Britain and France declared war on Germany, marking the beginning of World War II.