September 3: Allied invasion of Italy: Mainland Italy is invaded by Allied forces under General Sir Bernard Montgomery, for the first time in the war.

September 5: The 503rd Parachute Regiment under American General Douglas MacArthur lands and occupies Nadzab, just east of the port city of Lae in northeastern Papua New Guinea.

September 7: Gulf Hotel fire: A fire at the Gulf Hotel in Houston, Texas, kills 55.

September 8: United States General Dwight D. Eisenhower publicly announces the surrender of Italy to the Allies. - Frascati bombing raid: The USAAF bombs the German General Headquarters for the Mediterranean zone. - The first classes commence at Grace University in Omaha, Nebraska.

September 9: Bertolt Brecht's play Life of Galileo (German: Leben des Galilei) receives its first theatrical production at the Schauspielhaus Zürich.

September 12: Gran Sasso raid - German paratroopers rescue Mussolini from imprisonment, in Unternehmen Eiche ("Operation Oak").

September 16: The Salerno Mutiny occurs when soldiers of the British Army's X Corps refuse postings to new units.

September 17: Villefranche-de-Rouergue Mutiny - a group of pro-Partisan soldiers led by Ferid Dzanic' and others within the 13th Waffen Mountain Division of the SS Handschar (1st Croatian) training in Occupied France rise against Nazi German troops in the Division; the revolt is rapidly suppressed.

September 23: The Italian Social Republic ("Republic of Salò") is founded in northern Italy as a puppet state of Nazi Germany.

September 27: Four days of Naples begins: a popular uprising drives German occupying forces from the city. (source: Wikipedia)