Shortly after wiping Czechoslovakia off the map, Hitler shifted his attention to Poland, Germany’s eastern neighbor. Through Foreign Minister Ribbentrop, Hitler made clear to the Polish government that Germany demanded the city of Danzig, as well as the right to build a railroad and superhighway through the Polish Corridor, which separated East Prussia from the Reich. Poland refused, setting the stage for Hitler’s next act of unprovoked aggression, which would trigger World War II.

Hitler’s invasion of Bohemia and Moravia on March 15 left Poland vulnerable to German armies from the west, north, and south. With the Soviet Union to the east, Poland suddenly found itself surrounded by historically hostile powers that were now controlled by totalitarian dictators. On March 23, 1939, Hitler’s forces seized the Lithuanian port city of Memel. Eight days later, Prime Minister Neville Chamberlain—in a radical departure from his past behavior—guaranteed British and French support in the event of a German attack on Poland. On April 3, Hitler ordered his armed forces to prepare “Case White,” a plan to launch an invasion of Poland as early as September 1 (the date of the actual attack, as it turned out).