by Canada. September 13–14 – WWII: Zambrów massacre – German Wehrmacht soldiers shoot more than 200 Polish prisoners of war. September 15 – WWII: 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 Battles of Khalkhin Gol, the undeclared border war between the Soviet Union (and Mongolian allies) and Japan. September 17 – WWII: The Soviet Union invades Poland, and then occupies eastern Polish territories. Royal Navy aircraft carrier is torpedoed and sunk by in the Western Approaches with the loss of 519 crew (the first British warship loss of the war). September 18 – WWII: Orzeł incident: Polish submarine ORP Orzeł escapes internment from Tallinn Harbour, Estonia, leading both the Soviet Union and Germany to question Estonia's neutrality. September 19 – WWII: The Poznan pocket collapses, and the Germans capture, according to many sources, over 150,000 men. Many elements of General Tadeusz 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. Assassination of Armand Călinescu: Prime Minister of Romania Armand Călinescu is shot in Bucharest by members of the fascist Iron Guard. WJSV broadcast day: Radio station WJSV in Washington, D.C. records an entire broadcast day for preservation in the United States National Archives. September 22 – WWII: A joint victory parade is staged by the Wehrmacht and Red Army in Brest-Litovsk, at the end of the Invasion of Poland. September 24 – WWII: The Soviet Union issues an ultimatum to Estonia to allow Soviet military bases on its territory, which Estonia accepts on September 28. Similar ultimatums are issued to Latvia on October 5 and to Lithuania on October 10, who are forced to accept them as well. September 28 – WWII: 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 30 – General Władysław Sikorski becomes Prime Minister of the Polish government-in-exile. October October 6 – WWII: The Battle of Kock ends