The Western Army engaged various local forces from the Baltic States, Belarus, Poland and Ukraine, and its actions contributed to starting the Polish–Soviet War of 1919–1920.

The army fought in the Polish–Soviet War under the command of Nikolai Sollogub and advanced westwards into Poland in July 1920 before being thrown back during the Battle of Warsaw in August.

(His official rank was "military leader" (военный руководитель), since he was a former Tsarist general) The Western Curtain covered over 800 kilometres along the line Nevel–Polotsk–Senno–Orsha–Mogilev–Zhlobin–Gomel–Novy Oskol.

[2] Immediately after its formation, on November 17, 1918, the Western Army started a bloodless advance, following the retreat of the German forces, in the direction of Belarus and Ukraine.

The purpose of the Russian westward offensive of 1918–1919 was to take control over the territory abandoned by the German Army retreating from the Ober-Ost theater of operations.

The attack began on May 14, but failed when the Soviet advance was unable to build momentum and the 15th Army was counterattacked and forced into a disorderly retreat at the beginning of June.

During the offensive, the army rapidly advanced through Belarus, capturing Igumen, Bobruisk, Minsk, Baranovichi, Slonim, and Vawkavysk.

[1] The 16th Army continued to advance westward, and on the night of August 1, the army attacked Polish general Wladyslaw Sikorski's Polesie Group at Brest-Litovsk after crossing the Western Bug, and captured the town, breaching the fourth and final Polish defensive line in front of Warsaw.

[6] The army was unable to exploit its success by quickly advancing on Warsaw, however, because a counterattack by Sikorski in the first week of August threw it back across the Western Bug.

[7] On August 8, Tukhachevsky issued his order for the capture of Warsaw, in which the 16th Army was to attack the city from the east, covered on its flank by the Mozyr Group of Forces.

Under pressure, the three southernmost divisions of the 16th Army began a precipitate retreat to the east, but then were attacked in turn by troops from the Assault Group.

By this time, most of the army had fled across the Bug towards Białystok, with only the 27th Division holding a couple of crossings to allow stragglers to pass through.

On August 22, Białystok was captured by the Polish 1st Legions Infantry Division, cutting off stragglers from the army and the Mozyr Group.

In exchange, the army received the 48th and 56th Rifle Divisions, bringing it back up to its strength before the Battle of Warsaw,[13][3] and its headquarters was located at Slonim.