The Polish–Soviet war erupted in 1919 in the aftermath of World War I. The root causes were twofold: a territorial dispute dating back to Polish–Russian wars in the 17–18th centuries; and a clash of ideology due to RSFSR's goal of spreading communist rule further west, to Europe (Soviet westward offensive of 1918–19). At that time both countries had just undergone transition: in 1918 Poland reclaimed independence after 123 years of partitions. In 1917 the October Revolution replaced the liberal, democratic Provisional Government, that had previously displaced the Tsar in Russia, with Soviet rule. The war ended with the Treaty of Riga in 1921, which settled the border issue and regulated Polish-Soviet relations until the German invasion of Poland in 1939.
Soviet Forces in early 1920
Red Army forces had won a recent series of major victories against the White Russians, defeating Denikin, and had signed peace treaties with Latvia and Estonia. The Polish front, formerly a distraction, became the most important war theater and majority of Soviet resources and forces were diverted into it. In January 1920, the Red Army began concentrating a 700,000-strong force near the Berezina River and in the Byelorussian Soviet Socialist Republic. The Red Army totaled 5,000,000, with additional millions of Russian recruits to draw from, but much of that force was still engaged in the civil war. This number of troops were far less virile than the number of weapons available, and only one in nine soldiers could be properly classified a fighting man. In the course of 1920, almost 800,000 Red Army personnel were sent to fight in the Polish war, of whom 402,000 went to the Western front and 355,000 to the armies of the South-West front in Galicia. The Soviet manpower pool in the West was estimated at 790,000. The Soviets had at their disposal much military equipment left by withdrawing German armies, and modern Allied armaments (including armoured cars, armoured trains, trucks and artillery) captured from the White Russians and the Allied expeditionary forces following their defeat in the Russian Civil War. With the new forces, Soviet High Command planned a new offensive in late April/May.
Bolshevik commanders in the Red Army's coming offensive would include Mikhail Tukhachevsky (new commander of the Western Front), Leon Trotsky, the future Soviet ruler Joseph Stalin, and the founder of the Cheka secret police, Felix Dzerzhinsky.
Polish Forces in early 1920
The Polish Army was made up of soldiers who had formerly served in the various partitioning empires, supported by inexperienced volunteers and recruits. Logistics were a nightmare, relying on whatever equipment was left over from World War I and could be captured. The Polish Army employed guns made in five countries, and rifles manufactured in six, each using different ammunition. Before the Battle of Warsaw the 1st Legions Infantry Division comprised three regiments, one of which was armed with German Mauser rifles, a second with French Lebel rifles and Berthier carbines, while the third used Russian Mosin–Nagant rifles. Each make of weapon took ammunition of a different caliber.
The Polish forces grew from approximately 100,000 in 1918 to over 500,000 in early 1920. By 20 August 1920, the Polish army had reached a strength of 737,767, so there was rough numerical parity between the Polish army and the opposing Soviet forces. Polish intelligence knew the Soviets had prepared for a new offensive; the Polish High Command decided to launch a preemptive attack. The plan for Operation Kiev was more ambitious than simply defeating the Red Army on Poland's southern flank. It aimed at creating a friendly government within the captured Ukrainian territories. Afterwards, General Józef Piłsudski and his Polish General Staff planned to reassign the 3rd Army to the north, where Polish planners expected a major Red Army counteroffensive.
Operation Kiev
Until April the Polish forces had been slowly but steadily advancing eastward. By early January 1920, Polish forces had reached the line of Uszyca–Płoskirów–Starokonstantynów–Szepietówka–Zwiahel–Olewsk–Uborć–Bobrujsk–Berezyna–Dyneburg. The new Latvian government requested Polish help in capturing Dyneburg, and it was, after heavy fighting (3 January – 21 January) by the Polish 1st and 3rd Legion Divisions under General Edward Rydz-Śmigły. The city was then handed to the Latvians, who viewed the Poles as liberators. By March, Polish forces had driven a wedge between Soviet forces in the north (Byelorussia) and south (Ukraine), capturing the towns of Mozyrz and Kalenkowicze and significantly disrupting Soviet plans for their early offensive.