Dunkirk: May 1940
Following the German invasion of Poland on September 1, 1939, Britain and France declared war on Germany. The British Expeditionary Force was posted at the French-Belgian border to prevent Germany from invading France. Between the two world wars, France had built the Maginot Line—formidable fortifications along its border with Germany. On May 10, 1940, Germany invaded the neutral countries of Belgium, Luxembourg, and the Netherlands in order to bypass the Maginot Line and to invade France where its defenses were weakest. British troops then moved into Belgium to try to stop the German advance toward France.