Why did Britain and France declare war on Germany?

France and Britain declared war on Germany on Sept 3, 1939 in response to Germany's invasion of Poland on Sept 1, 1939. France and Germany were allies, and they combined forces with many other allied countries to defeat Germany.

World War II began in 1939 and continued for six years until officially ending on May 8, 1945. Through the course of WWII, Adolf Hitler captured and enslaved Jewish residents in every country that his troops overthrew. Those who weren't enslaved were starved and eventually executed. It took combined forces to liberate the Jewish population from German concentration camps and to defeat Hitler.