- Sep 1 Battle of Megiddo starts, the final Allied offense during Sinai and Palestine campaign won by the Allies
- Sep 1 US troops land in Vladivostok, Siberia, stay until 1920
- Sep 3 5 soldiers hanged for alleged participation in Houston riot (or Camp Logan riot); in all 19 mutineers were executed.
- Sep 3 Allies forced Germans back across Hindenburg Line
US Men's National Championship
Sep 3 US National Championship Men's Tennis, Forest Hills NY: Robert Lindley Murray successfully defends title; beats Bill Tilden 6-3, 6-1, 7-5
- Sep 4 Jhr Ch Ruys de Beerenbrouck becomes 1st Dutch Catholic premier
- Sep 4 US troops land in Archangel, Russia, stay 10 months
- Sep 5 Decree "On Red Terror" is published in Russia
- Sep 5 Due to WWI, 15th Baseball World Series begins a month early; Boston Red Sox defeat Chicago Cubs, 1-0 at Comiskey Park
- Sep 9 Dutch government of Ruijs de Beerenbrouck forms
- Sep 10 Boston Red Sox and Chicago Cubs players threaten to boycott the World Series unless they are guaranteed $2,500 to the winners & $1,000 each for the losers
- Sep 11 Baseball World Series: Boston Red Sox beat Chicago Cubs, 2-1 at Fenway Park for 4-2 series win; Sox 5th WS title
- Sep 12 WWI: US forces launch an attack on German-occupied St Mihiel
- Sep 13 Train accident at Weesp Netherlands, kills 42
- Sep 14 WWI: Austria-Hungary sends a note to the Allies requesting peace discussions, but the Allies reject the offer
- Sep 20 Royal Dutch Blast furnace & Steel factory opens in Hague
- Sep 26 Beginning of the Meuse-Argonne Offensive, more than 1 million American soldiers in the largest and most costly offensive of WWI
- Sep 29 WWI: Allied forces score a decisive breakthrough of Hindenburg Line
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