Aisne: FIRST BATTLE OF THE AISNE until September 28 begins. By 0300 hours British 11th Brigade after 30-mile march scrambles across river at Venizel (3 miles (ca. 5 km) east of Soissons) by damaged bridge and finds Germans on heights above as do French 45th Division.
French I Corps retakes Reims. Bülow to command German right wing (First, Second and Seventh Armies), Einem replaces the ill Hausen in command of Third Army. BATTLE OF THE GRANDE COURONNE ENDS, French 74th Reserve Division alone has more than 5,140 casualties since August 24.
East Prussia: Russians evacuate Tilsit. Germans reach Wirballen and Suvalki across Russian frontier.
Poland and Galicia: Grodek falls as Austrians retreat over river San across 6 bridges under heavy artillery fire.
Hungary: Serbian First Army storm Jarak by river Sava but news from river Drina halts advance in the evening and leads to a methodical unimpeded retreat back to Serbia until September 14.
Serbia: Austrian XIII Corps crosses river Drina at Kuriachista in the night of September 12-13 and threatening the Jadar Valley from North. Beginning of a complex struggle for wooden, mountainous summits.
North Sea: First British submarine success. E9 (Horton), sinks small German cruiser Hela, 6 miles (ca. 10 km) south-southwest of Heligoland. E9 returns flying ‘skull-and-crossbones’ flag, beginning Royal Navy submarine patrol tradition.
Western Front: Storms wreck 4 aircraft of No 5 Squadron RFC at Saponay. Squadron of French 16th Dragoons captures German aviation park near Villers-Cotterets.
German New Guinea: Australian troops occupy Rabaul and Herbertshohe, formal takeover next day.
Russia: STAVKA cables War Ministry for more munitions.