On September 1, 1939, German troops invaded Poland. Britain and France declared war on Germany two days later. By the end of the month, Hitler's armies had overrun western Poland. Soviet armies occupied eastern Poland, and the two countries subsequently formally divided Poland between them. In April 1940, German forces conquered Denmark and Norway, and in May they struck at the Netherlands, Belgium, Luxembourg, and France. French and British troops offered ineffective resistance against the lightning-like strikes, or blitzkrieg, of German tanks and airplanes. A large part of the French army surrendered, and some 300,000 British and French soldiers were trapped at Dunkirk on the coast of northern France. …