The army was deployed between the 1st and 2nd Armies during the East Prussian Campaign of 17 August to 15 September 1914, covering the left flank of the 1st Army along with the 2nd Army. During the Russian retreat from East Prussia it defended the line of the Bobr River and covered the direction of Augustów and Grodno. The 1st and 10th Armies covered the right flank of the Northwestern Front during the Warsaw–Ivangorod Operation of 15 September to 25 October. General of infantry Thadeus von Sievers replaced Flug on 23 September. During the Łódź Operation of 29 October to 11 November, the troops of the 1st and 10th Armies covered the Mława direction on the right bank of the Vistula. In the subsequent offensive into East Prussia, the army was halted by strongly fortified and held German fortifications at the Masurian Lakes and was unable to capture them.