Britain and France responded by declaring war on Germany in World War II because of the aggressive actions taken by Adolf Hitler and the Nazi regime. Hitler had violated the Treaty of Versailles by rearming Germany, remilitarizing the Rhineland, and annexing Austria and Czechoslovakia. Additionally, Hitler's invasion of Poland in September 1939 was the final straw for Britain and France. Both countries had guaranteed Poland's security and independence and considered the invasion an act of aggression and a threat to European stability. Hence, they declared war on Germany to fulfill their obligations to Poland and to prevent further German expansion in Europe.
