October 1917

October 4 – WWI: Battle of Broodseinde near Ypres: British Imperial forces overpower the German 4th Army's defences.

October 12 – WWI: First Battle of Passchendaele: Allies fail to take a German defensive position with the biggest loss of life in a single day for New Zealand, over 800 of whose men and 45 officers are killed, roughly 1 in 1000 of the nation's population at this time.

October 12-19 – WWI: Operation Albion: German forces land on and capture the West Estonian archipelago.

October 13 – Miracle of the Sun at Fátima, Portugal.

October 15 – WWI: At Vincennes outside Paris, Dutch dancer Mata Hari is executed by firing squad for spying for Germany.

October 19
 
Dallas Love Field airport is opened.
Carl Swartz leaves office as Prime Minister of Sweden after dismal election results for the right-wing in the Riksdag elections in September. He is replaced by liberal leader and history professor Nils Edén.

October 23 – A Brazilian ship is destroyed by a German U-Boat, encouraging Brazil to enter World War I.

October 25 (O.S.) (November 7, N.S.) – Traditional beginning date of the October Revolution and Russian Civil War.

October 26 – WWI: Brazil declares war against the Central Powers.

Brazilian President Venceslau Brás signs a declaration of war against the Central Powers

October 27 – WWI: Ottoman force attacks Desert Mounted Corps units garrisoning el Buqqar ridge during the Battle of Buqqar Ridge fought in the last days of the Stalemate in Southern Palestine.

 
October 31 – WWI: Battle of Beersheba: The British XX Corps and Desert Mounted Corps (Egyptian Expeditionary Force) attack and capture Beersheba ending the Stalemate in Southern Palestine.