On 2 (15) July 1915, the offensive of Mackensen’s group resumed. After heavy fighting, the Russian troops retreated to the Ivangorod—Lublin—Kholm line. On 9 (22) July 1915, German forces forced the Vistula north of Ivangorod (at the mouth of the Wieprz). A Russian counterblow delayed the enemy’s advance, but on 22 July (4 August) 1915 Warsaw and the Ivangorod fortress had to be abandoned. On 7 (20) August 1915, the Novogeorgievsk fortress fell (at the mouth of the Narew).
The Russians managed to avoid encirclement and capitulation; the only exception was the garrison of the Novogeorgievsk fortress, which did not receive in time from the Commander-in-Chief an order to leave the fortress. General Alekseev anticipated Hindenburg’s plan to execute an envelopment of the right flank of the Russian forces from the Neman River and strengthened the Vilna direction. In connection with the enemy strike that began from the Narew, the Russian Supreme Command decided to withdraw to the Lomzha—Ostrov—Opalin line, and then to the Osovets—Wlodawa line.
According to the Stavka directive of 4 (17) August 1915, the Northern Front was separated from the Northwestern Front and was tasked with covering the routes to Petrograd from East Prussia and from the Baltic Sea direction. On 9 (22) August 1915, Russian troops left the Osovets fortress (evacuating the garrison and mining the approaches to the fortress and the fortress itself); on the same day the Kovno fortress was abandoned; on 13 (26) August Brest-Litovsk and Olita were evacuated; and on 2 September Grodno was also abandoned after fighting.
On 23 August, Nicholas II assumed the duties of Supreme Commander-in-Chief, and M. Alekseev replaced N. Yanushkevich as Chief of Staff of the Stavka of the Supreme Commander-in-Chief.