Germany: Ludendorff warns Admiral Scheer of plans for abandoning Flanders coast.
Somme – Battle of Epehy: British Third and Fourth Armies (1,488 guns and 300 MGs) attack from 0700 hours in heavy rain with 21 tanks on 16-mile front northwest of St Quentin; 6,800 Australians (1,260 casualties) advance 5,000 yards; capture 4,243 PoWs; 76 guns and 330 MGs and mortars in total of 9,000 and 100 guns. British 12th and 58th Divisions capture Epehy.
North Russia: Karelian troops beat German-led force at Ukhtinskaya and drive it back into Finland.
South Russia: Red Southern Front created with Stalin as Military Council Chairmen (first meets on September 28 and forms Eighth to Twelfth Armies).
Salonika – Anglo-Greek attack astride Lake Doiran with 231 guns: Battle of Doiran (until September 19) at 0508 hours vs Bulgar 9th Division with 122 guns, British enter Doiran town and take Petit Couronne with 777 PoWs but British 65th brigade (22nd Division) has only 200 survivors (30 poisoned by British gas) from attempt to storm Grand Couronne, (thrice-wounded commander 7th South Wales Borderers Lieutenant-Colonel Burges awarded Victoria Cross). Greek losses 1,232 men, British failure east of lake (900 Anglo-Greek casualties) due to grass wildfire started by Bulgar artillery. Bulgar Deputy C-in-C Todorov cables Hindenburg pleading for at least 6 German divisions. Serb cavalry reach Polosko.
Trans-Jordan: Lawrence blows his 79th rail bridge north of Nisib Station south of Deraa as Arab Army turns east.
Palestine: On Allenby’s east flank 53rd Division storms across Wadi-es-Samieh and advances 7 miles (night September 18-19).
Salonika: RAF sends 372 calls for artillery fire until September 19, only 1 aircraft lost and shoots down 2 aircraft.
Western Front: Rain and clouds restrict flying over Somme (until September 19), both sides’ aircraft losses much lower (September 17-20).