September 1, 1939, is the date commonly associated with the start of World War II. On this day, Nazi Germany invaded Poland, marking the beginning of widespread military conflict in Europe. The invasion prompted Britain and France to declare war on Germany on September 3, 1939. This event signaled the collapse of diplomatic efforts to contain Hitler's aggression and unleashed a global war that would last until 1945. The impact of World War II was profound, reshaping political boundaries, social structures, and global power dynamics, leading to the establishment of the United Nations and the beginning of the Cold War.