The 16th Army (Russian: 16-я армия, romanized: 16-ya armiya) was a field army of the Red Army during the Russian Civil War era. It was originally formed as the Western Army (Russian: Западная Армия, romanized: Zapadnaya Armiya) on November 15, 1918, by the Russian SFSR for the purpose of recovering territories lost by the Russian Empire during the First World War and establishing Soviet republics in those territories. 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. The army retreated east into Belarus and was disbanded in May 1921.

After the Treaty of Brest-Litovsk, the newly established Russo-German border was controlled on the Russian side, by the so-called Western Section of Curtain Troops (Западный участок отрядов завесы), or simply the Western Curtain. The curtain was a sparse and heterogeneous set of detachments. Its commander was Vladimir Egoryev. (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. Eventually the Western Curtain was arranged into seven detachments with over 20,000 troops. This number was actually very small in relation to the area it covered, and insufficient in case of any larger battle. Moreover, part of its manpower was moved to other bottlenecks of the Russian Civil War.

Eventually, further recruiting by the Red Army allowed a reorganization of the detachments of the Curtain into regular divisions, and the Western Curtain was further reorganized into the Western Defense Region (Западный район обороны). It was created by the Revolutionary Military Council order #3/2 on September 11, 1918, headquartered at Kaluga. The Region extended from Petrograd to the Western edge of the Southern Front, and was commanded by Andrei Snesarev.

After the Treaty of Brest-Litovsk was annulled by the Soviets on November 13, 1918 the Western Defense Region was transformed into the Western Army (November 15, 1918), garrisoned in Smolensk. It was composed of the:

By the end of 1918, the strength of the Western (16th) Army was around 19,000 men, but had little artillery or cavalry (8 guns and 261 horses total). Over the next few months the strength of the Army grew to 46,000 men due to conscription and the mobilization of Communist Party members. It was considered by the Soviet High Command to be one of the least important armies in that period.

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. Later the Soviet Western Army engaged various self-defence and militia groups from Belarus, Lithuania, Poland and Ukraine. Among scores of battles, the Battle of Bereza Kartuska on February 14, 1919 sparked the Polish–Soviet War.

The army became part of the Western Front on February 19, and was renamed the Lithuanian-Belorussian Army on March 13. With this redesignation, the army officially became the army of the Lithuanian-Belorussian Soviet Socialist Republic, a Soviet puppet state covering the territory of Lithuania and Belorussia. On June 9, it became the 16th Army after it was pushed out of Lithuania after retreating from Panevėžys in the Lithuanian War of Independence. Around the same time, the army headquarters was moved forward to Mogilev. The army continued to retreat until August, ceding Molodechno and Minsk to Polish troops, and in August took up defensive positions on the line of the Berezina River. From August 14, it was commanded by Nikolai Sollogub. In September, the army headquarters was relocated back to Smolensk, but moved to Novozybkov from March and May 1920. Between March and April 1920, the army fought to capture Mozyr. In May, the headquarters moved back to Mogilev.

From May 14 to June 8, 1920, the army fought in the Western Front counterattack against the Polish Kiev offensive, the May Offensive. In the counterattack, planned by front commander Mikhail Tukhachevsky, the 16th Army was to frontally attack the Polish 4th Army at Borisov and Igumen and keep it from moving to reinforce threatened Polish units, while the 15th Army, the main force, would advance southwest towards Molodechno, then wheel around and push the 1st Polish Army into the rear of the 4th, driving them into the Pripet marshes and destroying them there. 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. The 16th Army crossed the Berezina and created a bridgehead, but was itself counterattacked and forced to retreat back across the Berezina.