However, Pilsudski launched his attack, known as the Battle of the Niemen River, on September 21. Although the Soviet line initially held, Polish forces moved through Lithuania, then swept south into the rear of the Soviet northern flank, which began a disorganized retreat. In the center, the 15th and 16th Armies retreated in an orderly fashion thanks to the actions of the 27th Division, the southernmost of the 15th Army units. By October 1, the units of the two armies were holding positions in a line of old Russian World War I trenches, opposite the former Polish positions from before the July Offensive. The defense line was quickly broken through by the Polish advance, and by the time the Polish troops captured Minsk and reached the Berezina on October 15, the 16th Army had been reduced to a skeleton of its original size. The war ended on the next day when the cease-fire between Poland and the RSFSR took effect. On October 17, the army was reinforced with troops from the disbanded 4th Army. From October, it was headquartered at Mogilev. Between November and December, the 16th Army fought against Stanisław Bułak-Bałachowicz's troops. On December 7, the army received the troops of the disbanded 3rd Army. In March, the Gomel Fortified Region became part of the 16th Army. The army was disbanded on May 7, 1921, and its troops distributed to other Western Front units.