September 1: Beginning of WWII. - Opening shots of World War II and Invasion of Poland: At 4:45am Central European Time, under cover of darkness, the German WW1-era battleship Schleswig-Holstein quietly slips her moorings at her wharf in Danzig harbor, drifts into the center of the channel, and commences firing on the fortress Westerplatte, a Polish army installation at the mouth of the port of Danzig, Poland. Simultaneously, shock-troops of the German Wehrmacht begin crossing the border into Poland. - The Reichstag passes a statement stating that Hitler's second-in-command Reichsmarshall Hermann Göring should be appointed as Hitler's successor as Führer should Hitler die during the War. Rudolf Hess is to be appointed in Göring's place should anything befall Göring. - Norway, Finland, Sweden and Switzerland declare their neutrality.
September 2: Following the invasion of Poland, Danzig (now Gdan'sk, Poland) is annexed to Nazi Germany. - Spain and Ireland declare their neutrality.
September 3: The United Kingdom, France, New Zealand and Australia declare war on Germany. - President Franklin Delano Roosevelt advocates neutrality in a Nation-wide radio address. - British liner SS Athenia becomes the first civilian casualty of the war when she is torpedoed and sunk by German submarine U-30 in the eastern Atlantic. Of the 1,418 aboard, 98 passengers and 19 crew are killed. - Canadian Prime Minister Mackenzie King, in English, and Justice Minister Ernest Lapointe, in French, give an international radio address, stating its intentions to declare war against Nazi Germany.
September 4: Nepal declares war on Germany.
September 5: The United States declares its neutrality in the war.
September 6: South Africa declares war on Germany.
September 8: Little Sisters of Jesus founded in Algeria by Little Sister Magdeleine. - Forward elements of General Hoeppner's XVI Panzerkorps take up positions outside Warsaw. The world is stunned by the rapidity of the German advance and the Polish High Command is effectively isolated, but lack of infantry support and effective civilian resistance cause Hoeppner to halt outside the city itself. - Polish troops on the Westerplatte are forced, due to lack of food and ammunition, to surrender. The garrison of about two hundred had held out against thousands of German forces (many of them Naval officer cadets from the Schleswig-Holstein,) for seven days.
September 9: Troops of the Polish Poznan Army under the command of General Kutrzeba open the Battle of the Bzura, the largest and best organized counter-attack mounted by the Polish forces in the campaign of 1939. For the first few days all goes well and the Germans are forced to retreat; but quick reaction by mechanized units and the Luftwaffe soon take their toll and the operation bogs down.
September 10: Canada declares war on Germany.
September 15: Diverse elements of the German Wehrmacht surround Warsaw and demand its surrender. The Poles refuse and the siege begins in earnest.
September 16: A ceasefire ends the undeclared Border War between the Soviet Union (and Mongolian allies) and Japan.
September 17: The Soviet Union invades Poland and then occupies eastern Polish territories. - Royal Navy aircraft carrier HMS Courageous is torpedoed and sunk by German submarine U-29 in the Western Approaches with the loss of 519 crew, the first British warship loss of the War.
September 19: The Poznan pocket collapses, and the Germans capture, according to many sources, over 150,000 men. Many elements of General Kutrzeba's forces work their way into Warsaw under extreme difficulty.
September 21: Reinhard Heydrich, chief of the Security Police, sends a directive, the Schnellbrief, explaining that Jews living in towns and villages in the Polish occupation zones are to be transferred to ghettos, and Jewish councils, Judenräte, will be established to carry out the German authorities' orders. - Radio station WJSV in Washington, D.C. records an entire broadcast day for preservation in the National Archives.
September 22: Joint victory parade of Wehrmacht and Red Army in Brest-Litovsk at the end of the Invasion of Poland.
September 28: Nazi Germany and the Soviet Union agree on a division of Poland after their invasion. - Warsaw surrenders to Germany; Modlin surrenders a day later; the last Polish large operational unit surrenders near Kock 8 days later.
September 29: Gerald J. Cox, speaking at an American Water Works Association meeting, becomes the first person to publicly propose the fluoridation of public water supplies in the United States. (source: Wikipedia)