- The German 1st Army crosses the Gette River in Belgium today, and find that the Belgian army has retreated in the night. The Germans are furious that the Belgians have escaped destruction. Perhaps not coincidentally, the village of Aerschot, between Brussels and the Gette River, suffers the first mass execution - 150 civilians are rounded up and shot.
- The French Army of Alsace, operating between 1st Army and the Swiss border, recaptures Mulhouse today. However, its commander, General Paul-Marie Pau, is suspicious of the combat effectiveness of the reserve divisions, and otherwise restricts his activity to securing the Vosges, despite the fact that he is directly opposed by only four German Landwehr divisions. Pau's inactivity allows the German 7th Army to threaten the right flank of the French 1st Army, which pulls the latter southward to cover the threat. This in turn further draws the French 1st Army away from the French 2nd Army.
- Still ten miles east of Gumbinnen, François argues to Prittwitz that the Russian 1st Army should be attacked again - the Russians were disorganized by their advance and prior fighting, insisted François. Prittwitz knew that initiating battle east of the Angerapp would mean abandoning the defensive positions the Germans had established there. On the other hand, the Germans had intercepted a message from General Paul von Rennenkampf, commander of the Russian 1st Army, to one of his corps commanders, ordering that the army will rest on the 20th to bring up supplies and restore cohesion. Prittwitz worries that if the Russian 1st Army delays its advance to the Angerapp, there will not be enough time to defeat them and redeploy southwestwards against the Russian 2nd Army. Prittwitz decides instead to attack along the lines suggested by François - his corps will attack tomorrow morning, while the two and a half corps along the Angerapp will march east to join.
- The main Austro-Hungarian offensive against Russia begins today, with 1st and 4th Armies advancing north from Galicia into Russian Poland, in the direction of Lublin and Cholm. Conrad's plan, to the extent that he has one, is to cut off the western portion of Russian Poland, isolating and destroying the Russian armies there. In this maneouver he had hoped to be joined by a German force advancing south from East Prussia as the other half of the pincer. Moltke has since disabused Conrad of this notion, yet Conrad persists with this operation. Moreover, the position of 1st and 4th Armies will worsen the further north they go. The Galician frontier resembles a half moon, and its length expands as the front moves into Russia. Thus the flanks of 1st and 4th Armies, and in particular the right flank of 4th Army, arrayed to the east of 1st Army, will be exposed. To the south, 3rd Army and whatever elements of 2nd Army that arrive are undertake an 'active defense' of the eastern portion of Galicia. In practice, the commander of 3rd Army interprets this as a full advance into Russia. This leaves 3rd Army even more exposed than 1st and 4th Armies, with the added complication that its lesser size makes it much less capable of meeting whatever Russian resistance it encounters.
- In response to the defeat suffered on the night of the 16th, the commander of the Austro-Hungarian 5th Army orders VIII Corps to retreat behind the Drina River. While a necessary decision in light of the losses of VIII Corps - one division alone had lost a third of its strength on the 16th - it leaves the other corps of 5th Army - XIII Corps - unsupported in Serbian territory.
With the growing realization of the failure of 5th Army's offensive, Potiorek's constant demands of Conrad to allow more substantial use to be made of 2nd Army bears limited fruit - Conrad agrees to temporarily transfer IV Corps to 5th Army, and the unit launches an attack from the Sabac bridgehead against the Serbian 2nd Army. The offensive makes some headway, but just as the Serbs appear about to give way, Conrad orders the effort called off, to allow for transfer of the corps to Galicia.