- Sep 1 Lord Kitchener arrives in Paris
- Sep 1 St Petersburg, Russia changes name to Petrograd
- Sep 1 The last passenger pigeon, a female named Martha, dies in captivity in the Cincinnati Zoo (Cincinnati, Ohio)
US National Men's Championship
Sep 1 US National Championship Men's Tennis, Newport, R.I.: R. Norris Williams wins his first major title beating defending champion Maurice McLoughlin 6-3, 8-6, 10-8; final event at Newport Casino, moves to Forest Hills, NY
- Sep 2 -3] Gen von Hausen & countess of France regime flees to Bordeaux
- Sep 2 The US Treasury Department establishes the Bureau of War Risk Insurance to provide up to $5 million worth of insurance for merchant ships and their crews
- Sep 3 British expeditionary army/general Lanrezacs army attack the Marne
Pope Benedict XV
Sep 3 Cardinal Giacome della Chiesa becomes Pope Benedict XV
- Sep 3 French troops vacate Rheims
- Sep 3 Lemburg capital of Galicia, is taken after a three-day battle in which the Russians rout the Austrians
- Sep 3 Prince Wilhelm von Wied leaves Albania
- Sep 4 France, Russia, and Britain agree in a Pact of London that none will make a separate peace
- Sep 4 General von Moltke ceases German advance in France
- Sep 5 French headquarters move to Chatillon-sur-Seine
- Sep 5 Great Britain, France, Belgium and Russia sign Pact of London
- Sep 5 Proclamation prohibits Canadian mint from issuing gold coins
Transatlantic Communications
Sep 5 US President Woodrow Wilson orders US Navy to make its wireless stations accessible for any transatlantic communications - even to German diplomats sending coded messages; leads to interception of the Zimmermann telegram, helping bring the US into the war
First Battle of the Marne
Sep 6 World War I: First Battle of the Marne begins, French and British forces prevent German advance on Paris (till the 12th Sept)
- Sep 7 NY Post Office Building opens to public
- Sep 8 HMS (formerly RMS) Oceanic, two weeks into its service with the Royal Navy, runs aground off, Foula, Shetland, Scotland in good weather and eventually sinks
- Sep 8 Private Thomas Highgate becomes the first British soldier to be executed for desertion during WW1
- Sep 9 -12] Belgian offensive from Antwerp
- Sep 9 Boston Brave George Davis no-hits Philadelphia Phillies, 7-0
- Sep 9 First fully mechanized unit in the British Army created - the Canadian Automobile Machine Gun Brigade (WWI)
- Sep 9 Meeting held at Gaelic League headquarters between Irish Republican Brotherhood and other extreme republicans; initial decision made to stage an uprising while Britain is at war
- Sep 11 Australia invades New Britain, defeating a German contingent there.
St Louis Blues
Sep 11 W. C. Handy "Father of the Blues" publishes his most famous composition "St Louis Blues"
Your Country Needs You
Sep 14 Lord Kitchener: "Your country needs you" appears as front cover design for the London Opinion magazine
- Sep 15 First Battle of Aisne finishes, Germans vs. French & British during WWI
- Sep 15 US Marines march out of Vera Cruz, Mexico
Australian Election
Sep 17 Andrew Fisher becomes Prime Minister of Australia for the third time
- Sep 18 Battle of Aisne ends with Germans beating French during WW I
Hindenburg German Commander
Sep 18 General Paul von Hindenburg named commander of German armies on the Eastern Front
- Sep 18 Irish Home Rule bill receives Royal assent
- Sep 18 South African troops land in German South West Africa
- Sep 19 Brooklyn Tip-Tops' Ed Lafitte no-hits Kansas City Packers (Federal League), 6-2
- Sep 20 John Redmond urges Irish Volunteers to enlist in the British Army
- Sep 22 1 German submarine sinks 3 British ironclads, 1,459 die
Louis Botha Takes Command
Sep 22 Prime Minister of the Union of South Africa, Louis Botha, assumes command of the armed forces after dismissing General Christiaan Frederik Beyers for his resistance to aid the British in the war against Germany
- Sep 23 Reds drop their 19th straight game, then beat Boston Braves
- Sep 26 Federal Trade Commission formed to regulate interstate commerce
- Sep 26 Germans arrest A Max, mayor of Brussels
- Sep 27 Cleveland 2nd baseman Nap Lajoie collects his 3,000th hit
- Sep 28 German forces move into Antwerp, Belgium (WWI)
- Sep 29 Boston Braves, who were in last place in mid-July, clinch NL pennant
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