World War II began on this date when Nazi Germany, under Adolf Hitler, invaded Poland. This act of aggression prompted the United Kingdom and France to declare war on Germany on September 3, 1939, marking the official start of the war in Europe. The invasion was a direct result of the Molotov-Ribbentrop Pact between Germany and the Soviet Union, which divided Poland between the two powers.

Following a series of military victories, Germany invaded and conquered France in June 1940. This led to the establishment of the Vichy French government and marked a significant expansion of German control over continental Europe. The fall of France shifted the focus of the war, with the British Empire continuing to resist German forces, particularly in the aerial Battle of Britain and the naval Battle of the Atlantic.