September 1917

September 14 – Russia is declared a republic by the Provisional Government.
 
September 23 – Leon Trotsky is elected Chairman of the Petrograd Soviet.
 
September 25 – The Mossovet (Moscow Soviet of People's Deputies) votes to side with the Bolsheviks.

September 26–October 3 – WWI: Battle of Polygon Wood (part of the Battle of Passchendaele) near Ypres in Belgium: British and Australian troops capture positions from the Germans.

September 28–29 – WWI: Second Battle of Ramadi: British troops take Ramadi from the Ottoman Empire.